The Amazon-Influenced Muddy Coast of South America: a Review of Mud-Bank–Shoreline Interactions

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To link to this article : doi:10.1016/j.earscirev.2010.09.008 URL : http://dx.doi.org/10.1016/j.earscirev.2010.09.008

To cite this version : Anthony, Edward J. and Gardel, Antoine and Gratiot, Nicolas and Proisy, Christophe and Allison, Mead A. and Dolique, FrancK and Fromard, François The Amazon-influenced muddy coast of South America: A review of mud-bank–shoreline interactions. (2010) Earth-Science Reviews, vol. 103 (n° 3-4). pp. 99- 121. ISSN 0012-8252

Any correspondance concerning this service should be sent to the repository administrator: [email protected] The Amazon-in ﬂuenced muddy coast of South America: A review of mud-bank –shoreline interactions

Edward J. Anthony a,⁎, Antoine Gardel b, Nicolas Gratiot c, Christophe Proisy d, Mead A. Allison e, Franck Dolique f, François Fromard g a Aix Marseille Université, CEREGE, UMR CNRS 6635, Europôle Méditerranéen de l'Arbois, B.P. 80, 13545 Aix en Provence Cedex, France b Université du Littoral Côte d'Opale, Laboratoire d'Océanologie et de Géosciences, CNRS UMR 8187 LOG, 32, Avenue Foch, 62930 Wimereux, France c IRD/UMR LTHE, Laboratoire d'étude des Transferts en Hydrologie et Environnement, BP 53, 38 041 Grenoble Cedex 9, France d IRD/UMR AMAP, botAnique et bioinforMatique de l'Architecture des Plantes, TA A51/PS2, Boulevard de la Lironde, 34398 Montpellier Cedex 5, France e University of Texas Institute for Geophysics, John A. and Katherine G. Jackson School of Geosciences, J.J. Pickle Research Campus, Bldg. 196 (ROC), 10100 Burnet Road (R2200), Austin, TX 78758-4445, USA f Université Antilles-Guyane, Campus de Martinique, BP 7207, 97275 Schoelcher Cedex, France g CNRS, UPS, INP, EcoLab - Laboratoire d'écologie fonctionnelle UMR 5245, 29 rue Jeanne Marvig, 31055 Toulouse, France

abstract

The 1500 km-long coast of South America between the Amazon and the Orinoco river mouths is the world's muddiest. This is due to the huge suspended-sediment discharge of the Amazon River (10 6 ×754tons yr −1 ±9%), part of which is transported alongshore as mud banks. Mud-bank formation is controlled by the physical oceanography of the continental shelf seaward of the Amazon River mouth, an initial sea floor storage area for much of the suspended sediment discharged from the river. In this area, rapid and sustained fluid-mud Keywords: concentration and trapping are associated with fresh water –salt water interaction and estuarine front activity on Amazon 3 −1 mud banks the shelf due to the enormous Amazon water discharge (ca. 173,000 m s at Obidos, 900 km upstream of the muddy coast mouth). Fluid mud is transported shoreward and then along the coasts of the Guianas by a complex interaction of mangroves wave and tidal forcing, and wind-generated coastal currents. The mud banks, which may number up to 15 or wave dissipation more at any time, are up to 5 m-thick, 10 to 60 km-long, and 20 to 30 km-wide, and each may contain the cheniers equivalent mass of the annual mud supply of the Amazon. As the banks migrate alongshore, their interaction with beach rotation waves results in complex and markedly fluctuating shorelines that are associated with space- and time-varying South America depositional ‘bank ’ phases and erosional ‘inter-bank ’ phases. Bank zones are protected from wave attack as a result of wave-energy dampening by mud, and undergo signi ficant, albeit temporary, coastal accretion accompanied by rapid mangrove colonization. The dampening of waves in bank areas as they propagate onshore is accompanied by the shoreward recycling of mud, commonly in the form of individual mud bars. These bars progressively undergo desiccation and consolidation, and thus constitute a major pathway for rapid and massive colonization by mangroves. Erosion by waves propagating across relatively mud-de ficient shoreface zones in inter-bank areas can lead to muddy shoreline retreat rates of tens of metres to several kilometres over a few months to a few years, accompanied by massive removal of mangroves. Notwithstanding the higher incident wave energy on inter-bank shores, inter-bank shorefaces are permanently muddy due to the pervasive in fluence of the Amazon muddy discharge. Inter-bank and transitional bank-to-inter-bank phases are associated with both periodic sandy chenier formation and extreme forms of rotation of rare headland-bound sandy beaches. The high mud supply from the Amazon has been the overarching geological control on the Quaternary evolution of the northeastern South American coast, having led to the growth of a muddy shelf clinoform at the mouth of the Amazon and more or less important progradation throughout this coast. Net progradation re flects an imbalance in favour of deposition during each mud-bank –inter-bank cycle. The high mud supply has presumably blanketed shelf sand deposited by smaller rivers during eustatic lowstand phases. The shelf clinoform structure at the mouth of the Amazon and the muddy coastal progradation throughout the coast of the Guianas and into Venezuela provide analogues of the geological record on muddy shorefaces.

⁎ Corresponding author. E-mail address: [email protected] (E.J. Anthony).

doi: 10.1016/j.earscirev.2010.09.008 Contents

1. Introduction ...... 100 2. Amazon mud supply, mud-bank characteristics and environmental context ...... 102 2.1. Mud supply ...... 102 2.2. Mud-bank sediment concentrations, topography, internal structures and biogeochemical recycling ...... 103 2.3. Mud-bank oceanographic setting: winds, waves and currents ...... 105 3. Mud-bank formation ...... 105 4. Mud-bank morphosedimentary processes ...... 106 4.1. Wave –mud interactions ...... 106 4.2. Cross-shore mud dynamics and mud-bank ‘attachment ’ to the terrestrial shoreline ...... 106 4.3. Inter-bank zones ...... 107 5. Mud banks and mangrove dynamics ...... 108 6. Mud-bank migration and medium-term (order of years) shoreline dynamics ...... 111 7. Mud banks and sand bodies ...... 113 7.1. Mud-bank in ﬂuence on embayed bedrock headland-bound sandy beaches ...... 113 7.2. Cheniers ...... 115 7.3. Sand –mud interactions and sand deformation structures ...... 115 8. Discussion ...... 117 8.1. The mud-bank –inter-bank cycle and overall coastal-plain progradation ...... 117 8.2. Muddy shoreline progradation and large-scale clinoform development ...... 117 8.3. Comparisons with other muddy coasts ...... 118 9. Conclusions ...... 118 Acknowledgements ...... 119 References ...... 119

1. Introduction These studies have notably focussed on processes of mud-bank migration, mud-bank interaction with the shore, and the in fluence of Muddy shores and adjacent shorefaces are found along several such banks on mangrove dynamics. Commencing with the publication open coasts of the world. They are generally associated with the by Choubert and Boyé (1959) on variations between muddy shore dispersal pathways of rivers that discharge large quantities of fine- phases and phases of mud erosion attributed to sunspot activity, and grained sediment. While deltas constitute the typical setting for such an important grey-literature report by Nedeco (1968) , these efforts muddy coasts, longshore redistribution of sediment from high- were followed by the bench-mark publications of Augustinus (1978), discharge rivers may lead to the formation of signi ficant stretches of Wells and Coleman (1978, 1981a,b), and Froidefond et al. (1988) . muddy coasts downdrift of sediment source(s) ( Wright and Nittrouer, Further research on the coast of French Guiana and western Surinam 1995 ), and associated signi ficant clinoform development ( Walsh and was carried out within the framework of a French Guiana project Nittrouer, 2009 ). Examples include the Mississippi chenier coast (2000 –2004) on the dynamics of mud banks and their relationship (McBride et al., 2007 ), the Gulf of Papua ( Walsh and Nittrouer, 2004 ), with the mangrove-fringed shores, the results of which were the East China Sea downdrift of the Chanjiang/Yangtze ( Liu et al., published in a special issue of the journal Marine Geology in 2004 2006; Wei et al., 2007 ), and the Mekong delta coast ( Tamura et al., (Baltzer et al., 2004 ). The Amazon-in fluenced muddy coasts have 2010 ). The longest of these muddy coasts, however, is the 1500 km- continually attracted research efforts, notably regarding the interac- long stretch between the mouths of the Amazon and the Orinoco tions between mud and the hydrodynamic forcing, between mud and Rivers in northeastern South America ( Fig. 1 ), which is strongly shoreline, and between mud and mangroves. The aim of this review is impacted by the mud supply from the Amazon. This large supply of to bring together a comprehensive summary of the relationships mud constitutes the overarching geological control on this coast, between the hydrodynamics, mud supply and mud-bank formation, having led to the growth of a shelf clinoform structure at the mouth of and the morphodynamic interactions between mud banks and the the Amazon and more or less important coastal progradation shoreline , while also highlighting the impacts of mud-bank activity on throughout the coast of the Guianas and into Venezuela. By inducing the mangrove system. The efforts reviewed here depict a highly macroscale geomorphic and bathymetric changes, the high mud complex system characterized by strong estuarine frontal activity on supply also has considerable coastal ecological and economic impacts the continental shelf at the mouth of the Amazon that serves as a on all the countries between the mouths of the Amazon and the precursor to the mud-bank regime. Orinoco: Brazil, French Guiana, Surinam, Guyana and Venezuela. The recent findings examined in this research review have been Much of the earlier earth-science research on this coast concerned based on a wide range of methodological procedures and techniques, the mouth of the Amazon and the adjacent continental shelf including new field and remote-sensing approaches. Field studies on (summarised by Nittrouer and Demaster, 1986 ). Later, the AmaSSedS mud banks and on the muddy shoreline involve considerable (A Multidisciplinary Amazon Shelf Sediment Study) project ( Nittrouer logistical dif ficulties, including the challenge of accessing experimen- and Kuehl, 1995; Nittrouer et al., 1995a ) provided impetus for ground- tal sites. Field approaches have included: measurements of cross-shelf breaking research on the processes of fluid-mud accumulation and wave dissipation on mud beds ( Wells and Kemp, 1986 ), deployment fine-grained sedimentation on the coast of the Amapà area in the of instrumented tripods and seismic pro filing ( Allison et al., 2000 ), vicinity of the mouth of the Amazon ( Fig. 1 ). Efforts on the rest of this high-resolution in situ topographic mapping of a mud bank ( Lefebvre coast, dominated by mud-bank dynamics, from the Cabo Cassipore et al., 2004; Anthony et al., 2008 ), monitoring of mud-bank area through the three Guianas (French Guiana, Surinam and Guyana) sedimentation rates through ultrasonic altimetry ( Gratiot et al., to the Orinoco have been more sparse, but have nevertheless been 2007 ), and in situ instrumented monitoring and time-lapse photog- precursory in the understanding of patterns of behaviour, at various raphy to monitor changes in the surface properties of a mud bank timescales, of wave-dominated shores subject to high mud supply. (Gardel et al., 2009 ). Given the problems of accessibility, remote Fig. 1. A 2006 JERS-1 satellite image of the muddy Amazon –Orinoco coast, the world's longest muddy coast (a). Mud banks start forming in the Cabo Cassipore area rather than along the coast nearer to the mouth of the Amazon. This is due to the signi ficant offshore location of the non-con fined estuarine turbidity maximum of the Amazon and westward alongshelf de flection of this maximum towards the coast near Cabo Cassipore, directly south of which the Amapà coast appears to be mud-starved (see text). The oblique aerial photograph (b) shows a typical mud bank in French Guiana partly colonized by mangroves and cut by drainage channels. The bare part of the mud bank in the background shows a series of linear mud bars. This mud bank is one of several (up to 15 or more) banks migrating at any time from the mouth of the Amazon River in Brazil to that of the Orinoco River in Venezuela. sensing has been widely used for the analysis of coastal forms, interested in the hydrology of the Amazon River basin should consult physical processes, and mangrove ecology at various scales (e.g., the special volume edited by Guyot and Walling (2009) . Estimations Anthony et al., 2002; Allison and Lee, 2004; Baghdadi et al., 2004; of sediment discharge published over the last four decades have Froidefond et al., 2004; Baghdadi and Oliveros, 2007; Proisy et al., ranged from 5 to 13×10 8 m3 yr − 1, according to a review by Martinez 2007 ). In particular, topographic data extraction from sequential et al. (2009) . These authors obtained a mean annual mud discharge of satellite images has been used with success to highlight mud-bank about 754×10 6 tons yr − 1, with a coef ficient of variation of 8.6% dynamics and coastal evolution ( Gardel and Gratiot, 2004, 2005; (Fig. 2 ) from monitoring of a suspended-sediment discharge network Gratiot et al., 2008 ), while LIDAR has been shown to hold promise at Obidos run by the French –Brazilian research programme HYBAM for highlighting both topographic variations ( Anthony et al., 2008 ) (http://www.ore-hybam.org ) between 1995 and 2007. Martinez et al. and patterns of opportunistic mangrove colonization ( Proisy et al., (2009) highlighted a good agreement (to within 1.8 –3%) between 2009 ), although cost is a serious limitation given the extensive length their suspended-sediment concentrations and data derived from a of muddy shoreline. Radionuclide signatures have been used by MODIS space-borne sensor. The mean value obtained by these authors Allison et al. (1995a,b), Zhu et al. (2002), Aller et al. (2004), and appears to be a robust estimation of the suspended-sediment Allison and Lee (2004) to determine mud-bank history and dynamics, discharge of the Amazon compared to earlier estimations based on notably in situ mud residence times, and accumulation and recycling limited field measurements (e.g., Meade et al., 1985; Dunne et al., rates. 1998; Filizola, 2003 ). About 90% of the total sediment load of the The objectives of this review are to examine six issues on the Amazon is considered as being composed of silt and clay ( Milliman relationship between Amazon-derived mud and the evolution of the and Meade, 1983; Wright and Nittrouer, 1995; Dagg et al., 2004 ). Amazon –Guianas coast, and to discuss their relevance to studies of Bedload estimates are dif ficult to obtain. Strasser et al. (2002) − other, similar coastlines. These issues are: (1) a description of the computed a bedload discharge of 4.7×10 6 tons yr 1 from bedform mud-bank system and of the environmental context of this South structures, a value corresponding to 6% of the suspended load American coast preceded by data on the Amazon water and sediment computed by Martinez et al. (2009) . discharge; (2) mechanics of the estuarine interactions that lead to About 15 –20% of the muddy discharge progressively forms highly − mud-bank formation; (3) wave –mud-bank interactions and processes turbid suspensions ( N1 g l 1) in the vicinity of the mouth of the involved in cross-shore mud translation; (4) mud-bank interaction Amazon, and subsequently mud banks that migrate alongshore off the with mangroves; (5) the longshore migration of mud banks and its French Guiana –Surinam –Guyana coasts ( Augustinus, 1978; Wells and spatio-temporal variability; and (6) sand concentration processes and Coleman, 1978; Eisma et al., 1991; Allison et al., 2000; Warne et al., the in fluence of mud banks on sand bodies. The overall mud-bank 2002 ). In any year, the number of actively translating mud banks cycle that follows the original sequence of mud supply and throughout the 1500 km-long coast of the Guianas may be 15 or more. concentration, and mud-bank formation, migration, and interaction The banks are spaced at intervals of 15 to 25 km, are up to 5 m-thick, with the shore, and the geological signi ficance of muddy shoreface 10 to 60 km-long and 20 to 30 km-wide, and migrate at velocities − aggradation and shoreline progradation are then discussed, followed ranging from 1 to N5 km yr 1 (Gardel and Gratiot, 2005 ). They by a comparison with other high mud-supply shores. translate in water depths of b5 to 20 m over a modern inner shoreface mud wedge created from deposition from previous mud banks 2. Amazon mud supply, mud-bank characteristics and (Allison et al., 2000 ). The spatial (and temporal) imprint of the environmental context waxing and waning of mud-bank activity is characterized by ‘bank ’ and ‘inter-bank ’ phases, and locally by ‘transitional ’ phases ( Anthony 2.1. Mud supply and Dolique, 2004 ). Over time, the rhythmic nature of these alternating phases has an overwhelming impact on the coast, The Amazon is the world's largest river system with a drainage inducing rapid shoreline accretion and/or erosion, as well as basin of 6.1×10 6 km 2 (Organization of American States, 2005 ). The important ecological changes involving the development and de- mean annual water discharge at Obidos ( Fig. 1 ), 900 km upstream of struction of mangrove forests. The migrating mud banks tend to the mouth, has been estimated at 173,000 m 3 s− 1 by Martinez et al. imprint a northwestward de flection of the river mouths on the coast (2009) from continuous gauging between 1995 and 2007. Readers (Fig. 1 ). The updrift coastal sectors of these river mouths comprise

Fig. 2. Annual river and suspended-sediment discharge of the Amazon River at Obidos, 900 km upstream of the mouth, from 1996 to 2007. From Martinez et al. (2009) . large areas of water of normal oceanic salinity trapped inshore by the lower-salinity Amazon water advecting alongshore ( Lambs et al., 2007 ).

2.2. Mud-bank sediment concentrations, topography, internal structures and biogeochemical recycling

Fluid mud, the term most commonly used to evoke the rheology of mud banks, develops at concentrations at which the settling velocity of the mud particles starts to be impeded by inter-particle interactions, and has been described by Mehta (2002) as an energy-absorbing slurry with typical densities ranging from 10 to 300 g l −1. The densities of mud en route from the Amazon to the Orinoco are, in reality, extremely variable, the mud showing various stages of concentration and consolidation (Gratiot et al., 2007 ), depending on history, proximity to the shore, elevation within the tidal frame, and liquefaction processes. In bank phases, these stages range from very high suspended-sediment concentrations (1 –10 g l − 1), through fluid mud, to settled mud, − which, in turn, ranges from under-consolidated ( ≤650 g l 1) to consolidated sediment beds ( ≥750 g l − 1). Inter-bank zones are − generally associated with less turbid waters (SSC of b1–5 g l 1). Gratiot et al. (2007) have suggested that the km-scale mud-bank and inter-bank pro files conform to those of accretion- or erosion- dominated muddy shore pro files ( Kirby, 2000, 2002; Mehta, 2002 ). Inter-bank areas are characterized by receding, low and concave erosion-dominated pro files of consolidated mud (and sometimes chenier sands, see Section 7.2 ) while mud banks are characterized by prograding, high and convex accretion-dominated pro files of soft mud colonized by mangrove vegetation in the highest elevations, albeit with marked micro-scale topographic heterogeneity caused by variations in wave reworking and consolidation, dewatering processes, and drainage channels, especially near the terrestrial shoreline. The wave –seabed interaction patterns that lead to these two basic types of pro files are discussed in Section 4 . Lefebvre et al. (2004) highlighted, from a combination of aerial photographs and field monitoring, the presence of a narrow linear topographic high over a mud bank. Such linear features are clearly identi fiable from SPOT images and low- flying aircraft and are commonly dissected by channel networks ( Fig. 3 ). From the meshing of data from SPOT images, LIDAR, and high-resolution field monitoring, Anthony et al. (2008) have mapped the topography of a typical mud-bank surface near the contact with the terrestrial shoreline ( Fig. 4 ). The generated pro file shows a lower intertidal zone (below mean water level (MWL)) characterized by relatively regular linear bar features and an upper intertidal zone (above MWL) exhibiting a topography of highs and lows. Mud banks have been observed to contain abundant internal structures, but coring to observe such structures is only generally − possible when the mud is consolidated ( ≥750 gl 1). Fresh mud can form homogeneous beds commonly ranging from a few decimetres to over 1 m-thick. Rine and Ginsburg (1985) identi fied alternations of Fig. 3. SPOT image (a, 17 October 2006) and oblique aerial photograph (b, 18 December massive structureless mud beds up to as much as 2 m-thick, with beds 2006) showing linear bar features characterizing mud-bank topography (trailing edge of the Macouria bank in French Guiana). The bars are drained by tidal channels. Lines A often exhibiting parallel, wavy and lenticular laminations and, rarely, and B show locations of pro files generated from SPOT images in Fig. 4 . Lower left corner fi micro cross-lamination. Laminae of silt, and rarely ne sand, alternate of (a) shows erosion of the trailing edge of the bank, resulting in a flat consolidated bed with more clay-rich laminae, indicating grain-size sorting, and and an offset between the distal edge of the bar and the eroding proximal edge and the orientation of clay minerals is common ( Rine and Ginsburg, 1985; terrestrial shoreline. From Anthony et al. (2008) . Allison et al., 1995b ). Debenay et al. (2007) have highlighted the role played by diatom bio films in the formation of such laminations. These and preclude peat development despite the local abundance of internal structures are also well observed in areas where the shoreline mangrove leaf and propagule litter. undergoes erosion, resulting in exposure of consolidated beds that are Mud at concentrations below those of settled mud typically sometimes topped by fresh fluidized mud driven ashore ( Lefebvre et undergoes signi ficant and repeated remobilization by tides and waves al., 2004 ). Overall, the preservation of internal structures in these mud and is subject to diagenetic processes before its ultimate burial ( Zhu et banks very likely re flects high rates of sediment accumulation relative al., 2002; Aller et al., 2004; Allison and Lee, 2004; J.Y. Aller et al., 2010; to bioturbation ( Kuehl et al., 1996 ). As noted by Walsh and Nittrouer R.C. Aller et al., 2010 ). Aller et al. (2004) employed a broad range of 234 210 (2004) from a study of similar deposits in the Gulf of Papua, it is likely tracers such as Th ( t1/2 =24 days), Pb ( t1/2 =22 years), seasonal − that the large mud supply may lead to dilution of organic matter levels Cl pro files, and non-steady-state diagenetic models of pore-water Fig. 4. Digital elevation models and representative topographic pro files of a typical mud bank (Macouria mud bank, French Guiana, shown in Fig. 3 ) constructed from the meshing of data from: (a) SPOT images, (b) a LIDAR image, (c) a field survey. MHWL = Mean high water level; MWL = mean water level; MLWL = Mean low water level. From Anthony et al. (2008) . concentrations and oxidant-reductant relationships to demonstrate seasonally. In such areas, the sea floor, thus, acts as a massive “suboxic that mud banks are characterized by extraordinarily intense sedi- batch reactor ”, entraining and processing reactive marine plankton, mentary and biogeochemical recycling that considerably exceeds that regenerating Fe, Mn oxides, exchanging metabolites and nutrients of stable coastal systems, such as salt marshes, in material exchange with the oxygenated water column, and generating non-sul fidic with the sea. The upper 0.1 –1 m of deposits are reworked and authigenic minerals ( Aller et al., 2004 ). It has been suggested that exchanged with overlying water on timescales of b10 days to these conditions of intense biogeochemical recycling are favourable to the generation of biosphere diversity over geological time ( J.Y. Aller et Cabo Cassipore, 350 km northwest of the mouth of the Amazon al., 2010; R.C. Aller et al., 2010 ). (Fig. 1 ). The volume of each mud bank can contain the equivalent of the annual mud supply of the Amazon (i.e., 750 to 800×10 6 tons). 2.3. Mud-bank oceanographic setting: winds, waves and currents This, combined with the large number of mud banks migrating at any time, suggests that periodic bank formation is a multi-year process. The Amazon-in fluenced coast is affected by trade winds from the Martinez et al. (2009) showed that half the annual mud load (51% on northeast that are mainly active from January to May. These winds average over the period 1995 –2007) is discharged between January generate waves from an east to northeast direction ( Gratiot et al., and April, with little variability from year to year. These authors

2007 ). Waves have signi ficant periods ( Ts) of 6 to 10 s, and signi ficant highlighted, however, more signi ficant inter-annual variability in offshore heights ( Hs) of 1 to 2 m ( Fig. 5 ). Large swell waves generated sediment discharge, in contrast to the relatively regular Amazon by North Atlantic depressions in autumn and winter and by Central water discharge over the same period. There is a need for better Atlantic cyclones in summer and autumn are probably responsible for correlation of regional river basin water and suspended-sediment the longer-period waves ( N8 s). These longer waves have a directional discharge data, as have attempted, for instance, Gratiot et al. (2008) in range from north to north-northwest. The most energetic trade-wind their calculations of longshore mud budgets (see Section 6 ). waves are observed between December and April in response to peak Mud-bank formation is controlled by the physical oceanography of wind activity while swell waves appear to be most frequent in autumn the continental shelf seaward of the mouth of the Amazon, which is an and winter, reinforcing the relatively energetic winter to early spring initial sea floor storage area for much of the suspended sediment wave regime induced by the trade winds. Trade winds also generate discharged by the river ( Trowbridge and Kineke, 1994; Geyer and rains from December to July, with an intervening relatively dry month Kineke, 1995; Kineke et al., 1996; Geyer et al., 2004 ). These authors in March. The annual rainfall in the coastal zone varies from 2 to 3 m. have highlighted rapid and sustained fluid-mud concentration and Tides are semi-diurnal and the spring tidal range decreases from trapping associated with fresh water –salt water interaction and front macrotidal (up to 8 m) at the mouth of the Amazon, where the large activity over the shoreface, a precursor condition for the formation of shallow continental shelf induces tidal ampli fication, to microtidal to the mud banks. Speci fically, estuarine circulation taking place on the low-mesotidal (ca. 1.8 to 3 m) along the rest of the coast. Little is shelf instead of within the river mouth (due to the enormous water known of inshore tidal current patterns. Shore-normal tidal currents discharge) generates rapid sediment deposition on the shelf in water along the Guianas coast can locally reach 0.45 ms − 1 (Bourret et al., depths of about 20 –60 m. This sediment is then remobilized and 2008 ). In addition to energetic forcing by near-resonant semi-diurnal transported shoreward and then alongshore by a complex combina- tides and by large buoyancy flux from the Amazon River discharge, the tion of wave forcing, tidal currents, and wind-induced coastal shelf is also subject to stress from the northeasterly trade winds, currents. Nikiema et al. (2007) have shown, from coupling of a 3D resulting in strong along-shelf flow associated with the North Brazil hydrodynamic model with the bathymetry and the coastline, that a Current ( Geyer et al., 1996 ). strong coastal current associated with the mesoscale North Brazil Current generates permanent northwestward extension of the 3. Mud-bank formation sediment-charged Amazon plume, con firming earlier observations that attributed this net northwestward plume and ambient shelf The formation of distinct mud banks that migrate alongshore is a water motion to a large-scale pressure gradient associated with this predominant geomorphological characteristic of the Amazon-in flu- current system ( Geyer et al., 1996 ). Relaxations of this current due to enced coast of South America. The formation of such discrete banks mesoscale changes in wind intensity, as hypothesised by Eisma et al. suggests periodic (order of several years) and localized mud (1991) , and subsequently by Allison et al. (2000) , could be concentration mechanisms that are still not well understood. Allison responsible for the periodic formation of mud banks. Mud concen- et al. (2000) have shown that mud banks originate in the vicinity of trated in this frontal zone is then advected along the inner shelf west of the Amazon by waves and currents. Molleri et al. (2010) have a shown from satellite images of seawater salinity that the northwest- 2 ward flow of the Amazon plume occurs in a narrow coastal band from January to April, a period corresponding to the annual peak of both 1.5 mud discharge ( Martinez et al., 2009 ) and trade-wind and wave

(m) s activity.

H 1 Allison et al. (2000) highlighted a zone of relative water-column mud ‘de ficit ’ close to the northwestern approaches of the mouth of the J F M A M J J A S O N D Amazon south of Cabo Cassipore ( Fig. 1 ), and showed that the proto- month mud banks started forming in the vicinity of this muddy ‘cape ’. In this equatorial setting, the extension of the Amazon muddy plume is little b affected by the Coriolis force but is strongly modulated by the trade 10 winds. This plume extension presumably leaves behind the mud- fi 9 de cient zone between the mouth of the Amazon and Cabo Cassipore, where the estuarine front of the Amazon is de flected. Allison et al. 8 6

T (s) (1995b, 1996) have suggested that up to 150×10 tons of mud (ca. 7 20% of the annual mud discharge monitored by Martinez et al. (2009) ) may be stored in a year in the Cabo Cassipore area. Kuehl et al. (1996) J F M A M J J A S O N D identi fied periodic deposition and resuspension of seabed layers as month much as a metre thick over most of the inner shelf, shoreface and foreshore north of Cabo Cassipore. The strata formed as a result of this Fig. 5. Daily averages of wave-climate parameters concerning the Amazon-in fluenced process consisted of decimetre-thick mud beds separated by hiatal – coast of South America, Hs and Ts, derived from a 44-yr record (1960 2004) of the ERA- (scour) surfaces. These authors suggested that the volume of 40 (European ReAnalysis) wave dataset generated by the European Centre for Medium- sediment resuspended seasonally from the inner shelf surface layer Range Weather Forecasts (ECMWF) for the location 5° N, 52° W. Dots correspond to the first and third inter-quartiles, and circles to the median values. From Gratiot et al. is of the same order of magnitude as the annual input from the river, (2007) . indicating that resuspension is an important control on suspended- sediment distributions in shelf waters. Most resuspension from the frequency. Gratiot et al. (2007) suggested, from remote-sensing inner shelf surface layer occurs during November –May, a period that data and field observations, that waves over a French Guiana mud includes both autumn swell wave active and high winter to early bank did not deviate signi ficantly from the 2nd order Stokes theory up spring trade-wind stress. This resuspended sediment could contribute to about 5 m water depth (11 –13 km off-shore), but were rapidly to shoreface accretion north of Cabo Cassipore ( Kuehl et al., 1996 ), as dampened at water depths less than 1 m (6 –8 km offshore). This well to the sourcing of the mobile mud belt. dampening effect ( Fig. 6 ) has been shown in numerical wave models (Winterwerp et al., 2007; Rogers and Holland, 2009 ). van Ledden et al. 4. Mud-bank morphosedimentary processes (2009) showed, from a SWAN model analysis of a 3-day spate of high swell waves in Surinam, that while the mud banks signi ficantly Downdrift of the mouth of the Amazon, once mud banks are dampened the wave heights, they had almost no effect on the peak formed, the coastal sediment dynamics depend essentially on: (1) periods. interactions between the associated mud (both suspended and settled) and trade-wind-generated waves, incident swell waves, 4.2. Cross-shore mud dynamics and mud-bank ‘attachment ’ to the wave-generated currents, and tidal currents; (2) interactions be- terrestrial shoreline tween the mud and the shoreline; and (3) in situ changes associated with physical intertidal processes (and biogeochemical changes Observations carried out in French Guiana show that mud brie fly highlighted in Section 2.2 but not treated further in this mobilization is particularly marked following long periods of low paper), and with mangrove dynamics. Both field and remote-sensing wave forcing (essentially during the dry season from July to October, approaches are progressively illuminating these fine-scale interac- Fig. 5 ) and during neap tides. Following such periods of low wave tions, which involve wave-energy dampening, mud-bank liquefac- energy, even moderate-energy events, generally in autumn, can tion, cross-shore and alongshore mud advection, mud-bank generate signi ficant mobilization of mud ( Gratiot et al., 2007 ). consolidation, and mangrove colonization and removal. Periodic swell waves from the North Atlantic such as those reported by van Ledden et al. (2009) are expected to cause massive event-scale 4.1. Wave –mud interactions reworking of mud-bank sediments and of muddy inter-bank shores. The onshore arrival of fluid mud is, thus, generally hinged on such The formation of fluid mud leads to signi ficant and complex higher-energy pulses. interactions between the bottom and waves that are still not well Under strong wave action, fluid-mud advection shoreward against understood. Mehta (2002), Winterwerp et al. (2007) and Jaramillo et gravity occurs by Stokes' drift ( Gratiot et al., 2007; Winterwerp et al., al. (2009) propose a sequence of events involved in the erosion of a 2007 ). It is expected that such advection is largely one of fluid-mud visco-plastic muddy bed in response to increases in bottom stress flow, as opposed to upward entrainment of sediment into the water initiated by wave forcing. Cyclic pressures induced by incoming waves column, because most of the sediment mass tends to spread near the start by generating small elastic deformations within the seabed. As bottom. Near the terrestrial part of a mud bank, fluid-mud pushed these stresses exceed bed strength, internal failures commence, shoreward during the neap-to-spring cycle results in overall accretion resulting in the inception of bed liquefaction, a process reported by and increase in bank elevation. Wave mobilization of mud has also these and other workers (e.g., De Wit and Kranenberg, 1997 ) to be been inferred from remotely-sensed estimates of suspended partic- very rapid, on the order of tens of seconds and up to a few minutes at ulate matter using SPOT satellite imagery ( Froidefond et al., 2004 ). most. These processes generate a fluid-mud layer and an increase in From an 80-day record of bed-level and water-level changes in the sediment concentration towards the shoreline. As additional waves intertidal zone of a mud bank near Cayenne monitored using an come in, they generate internal waves at this lique fied mud –water ultrasonic altimeter coupled with a pressure transducer, Gratiot et al. interface and these are dissipated by internal friction within the mud (2007) identi fied a sequence wherein a 1 –3 m-thick mud layer layer. In reality, aspects of stress and liquefaction should depend on lique fied by the cyclic wave pressure gradients is transported en the intrinsic properties of the mud, its degree of consolidation, the masse shoreward by wave drift due to wave asymmetry. The wave climate, and water depth, as Rogers and Holland (2009) and mobilized mud layer formed a mud bar feature (see Figs. 3, 4 ) that Holland et al. (2009) have suggested from a combination of field was translated shoreward as gel-like fluid mud, especially when high measurements and modelling efforts of waves over a mud bed waves prevailed. associated with the shoreface of the Patos Lagoon estuary in southern Gratiot et al. (2007) further showed that the bars on South Brazil. American mud banks are formed from gel-like fluid mud in the In the cross-shore dimension, waves maintain the fluid mud in intertidal zone at locations where wave dampening is completed. suspension but wave heights decrease dramatically with distance Shore-normal tidal currents also probably contribute to shoreward shoreward due to energy dissipation. This important energy-damp- mud transport. Once wave action ceases, the muddy pro file may ening effect of thick mud beds on waves has been demonstrated by become more consolidated once again through gelling and under its Wells (1983) and Wells and Kemp (1986) , and con firmed on other own weight, but these processes, further discussed in Section 5 , muddy shorefaces, such as those of the Kerala coast of India ( Mathew require days to weeks. and Baba, 1995; Mathew et al., 1995; Jiang and Mehta, 1996; Tatavarti Linear shore-parallel bar accumulations are typical of wave-formed and Narayana, 2006; Narayana et al., 2008 ), and Louisiana ( Sheremet shore bodies, such as those commonly found in sandy beach and Stone, 2003; Jaramillo et al., 2009 ). Wells (1983) and Wells and environments ( Anthony, 2009 ). The fundamental difference here, Kemp (1986) measured a dissipation rate that grew from 88% to 96% however, is that these muddy features, unlike non-cohesive sand for wave heights at three muddy shoreface locations off the Surinam grains, are formed from wave drift of cohesive gel-like mud that coast. The water depths at this site decreased from about 7 to 3 m over becomes progressively consolidated in areas of complete wave a distance of about 7 km. These authors highlighted the rapid dissipation. Although these linear features generally occur as shore- dissipation of both short and longer-period waves, although the parallel bodies in the inner mud-bank areas near the terrestrial latter underwent greater dampening. Sheremet and Stone (2003) shoreline, bar-like features with an angular offset relative to the compared wave dissipation rates over sandy and muddy portions of terrestrial shoreline are observed at the eroding trailing edges of mud the Mississippi delta shoreface and observed wave heights 70% lower banks, where they are reworked by the obliquely incident northeast- over the muddy bed, and attributed this to enhanced attenuation. erly trade-wind waves ( Fig. 3 b). Successive bands of linear shore- They also noted that the dampening affected the entire wave parallel bars may re flect successive phases of wave-induced shoreward Fig. 6. Computed wave spectrum at three locations of the Amazon-in fluenced Demerara coast of Surinam showing progressive wave dampening shoreward over the muddy shoreface and changes in spectral frequency. From Winterwerp et al. (2007) . Solid and dotted lines show, respectively, conditions with and without locally generated waves. Note the different scales.

transport of mud under seasonal variations in wave energy, in mud bank and the shoreline in which the former is envisaged as a combination with neap-spring tidal range variations. The imprint of shore-welded feature (e.g., Augustinus et al., 1989 ). tidal range has been clearly highlighted by Gardel et al. (2009) from a 4-week-long time-lapse photographic monitoring experiment of a 4.3. Inter-bank zones mud bar surface during the 2008 equinoctial spring tides. The effects of seasonal variations in sea level also need to be invoked, although data Inter-bank areas associated with deeper shoreface zones are on these are lacking. subject to signi ficant shoreline retreat over timescales on the order The physical oceanographic measurements carried out by Gratiot of years. In these areas, wave breaking occurs directly on the shore, et al. (2007) support the suggestion by Allison and Lee (2004) , based and breaker heights increase as tidal range increases in the course of on radionuclides ( 7Be, 137 Cs, and 210 Pb signatures) in sediment cores the neap-to-spring tidal excursion. An inter-bank shoreline is from inner ( b5 m water depth) mud-bank areas, that wave-generated composed of either stiff mangrove-colonized consolidated mud that fluid-mud suspensions constitute the primary mechanism for deliv- may be rapidly eroded ( Fig. 8 a,c) resulting in a flat, furrowed bed, or of ering sediment across the intertidal zone of a mud bank, thus, sandy bodies that may be substantial enough to form coherent enabling accretion at the shoreline. 210 Pb and 14 C geochronology of beaches and cheniers (see Section 7.2 ) commonly subject to over- vibracores obtained by Allison and Lee (2004) from mud flats wash. Both chenier-free and chenier-bound muddy shorelines are − indicated rapid sediment accumulation (0.24 –2.0 cm yr 1) landward fronted by a shoreface of over-consolidated mud. On over-consoli- of the 2-m isobath, produced from a thick (50 –150 cm) seasonal dated shoreline mud, the erosion process may result in the breakage surface layer. These authors proposed a model in which the mud bank and transport of large mud clasts yielding mud ‘pebbles ’ away from is disconnected from the shoreline, and sediment reaches the upper the breaker zone. intertidal zone to generate shoreline accretion by fluid mud being The large-scale (1 –5 km) plan shape of the shore in inter-bank driven onshore during periods of coastal setup and flood tide ( Fig. 7 ). areas may sometimes comprise rhythmic alternations of shoreline This differs from an earlier conception of the relationship between the protuberances (megacusps) and embayments ( Fig. 8 b). The overall Fig. 7. Conceptual model of the dynamics of the inner part of a mud bank and its relationship with the terrestrial shoreline. From Allison and Lee (2004) . In this model, which differs from earlier models, the mud bank is disconnected and sediment reaches the upper intertidal zone to generate shoreline accretion by fluid mud driven onshore during periods of coastal setup and flood tides. Some of this sediment may return offshore during ebb-tide fluid-mud transport and/or mass flows. Arrows re flect the relative magnitude of sediment supply to the leading-edge deposition on the inner mud bank. The largest quantity is derived from erosion of the trailing-edge mangrove fringe, with additional material coming from erosion of the trailing edge and inter-bank intertidal –subtidal surface, and from updrift mud banks and the Amazon River. dynamics underlying these alternations of megacusps and bays are, activity. Such a complex mud-bank pro file re flects a primary control however, not known ( Lakhan and Pepper, 1997 ). We infer that their by waves. Other closely related in fluences include topographic regular spacing alongshore precludes control related to differential feedback on patterns of mud settling during the tidal excursion, consolidation levels of the mangrove-colonized substrate, as sug- consolidation processes due to evaporation and dewatering, dissec- gested by cusp horns associated with apparently resistant ‘headlands ’ tion by intertidal drainage channels, and colonization by mangroves. of mangrove. These features are probably the muddy equivalents of The linear bar-like features are formed, as indicated in Section 4.2 , sandy beach megacusps and embayments associated with infragravity from gradual accumulation of fluid mud inshore within a framework wave-energy cascades or with self-organised patterns of coastal of predominantly tidal modulation of the vertical excursion of wave morphology (e.g., Coco and Murray, 2007 ) that may develop from activity. These bars show marked cross-shore variations in the degree irregular initial alongshore variations in the resistance of over- of consolidation ( Fig. 9 ) that re flect three factors: (1) intertidal consolidated shoreline mud. elevation; (2) trapping of fluid mud in depressions between the bars; and (3) mud remobilization and fluidization by wave activity. Once in 5. Mud banks and mangrove dynamics the upper intertidal zone, the bars become immobilized over fairly long phases of low wave energy, and, thus, progressively dry out. This The mangrove-colonized shorelines of the Amazon-in fluenced involves changes in physical parameters, notably yield stress and coast of South America fluctuate at signi ficant short-term (order of pore-water salinity because of evaporation and dewatering ( Fiot and weeks to a few years) rates of several tens of metres to several Gratiot, 2006; Gardel et al., 2009 ). kilometres in the cross-shore direction. The dynamic ‘connection ’ of a Field measurements and remote-sensing observations suggest mud bank with the shore commonly creates an intertidal mud flat of that the bars have a feedback in fluence on subsequent patterns of several square kilometres in a few months, with very dense mangrove fluid-mud accumulation and channel development ( Anthony et al., development in a few years, followed by rapid erosion of mangroves 2008 ). In the course of the tidal excursion, the troughs isolated by and their substrate during the inter-bank phase. These processes have these bars are seen to trap high concentrations of suspended mud that created one of the most extensive and most sedimentologically progressively consolidates, protected from direct wave remobiliza- dynamic mangrove coasts in the world. The large mud supply also tion. The observations lead us to infer that mud remobilization can implies that in situ mangrove ecological dynamics are closely lead to marked spatial variability in fluid-mud concentration levels controlled by topographic changes brought about by mud redistribu- that are especially well expressed by bars in the lower intertidal zone tion. Smothering of pneumatophores and suffocation of older still subject to onshore mobility. The channels dissecting these bars mangroves commonly occur, for instance, as a result of fresh mud (Fig. 3 ) serve as drainage networks for ebbing tides, for water yielded inputs that are driven ashore from the bank ( Fromard et al., 1998; from dewatering of the fluid mud as it becomes consolidated, and for Anthony et al., 2008 ). rainfall. The combination of such drainage networks and variations in Where the mud banks are in such dynamic ‘contact ’ with the fluid-mud consolidation can generate decimetre-scale variations in shore, it has been shown that their surfaces may be characterized by the elevation of the mud bank at any given time, while channel marked topographic heterogeneity ( Anthony et al., 2008 ). On a typical dissection results in the substitution of the linear bar forms in the mud-bank surface, the innermost bar features, variably dissected by upper intertidal zone by more complex topography. There is a need, drainage channels, form a dynamic ‘suture ’ zone with the muddy however, for quanti fication of the sediment transport processes intertidal terrestrial shoreline ( Fig. 3 ). Fig. 3 shows, however, a clear operating over these bars. difference between a lower intertidal zone (below mean water level In the upper intertidal zone, rapid drying and compaction are (MWL)) characterized by relatively regular linear bar features, and an associated with the development of mud cracks and diatom bio films upper intertidal zone (above MWL) exhibiting an intricate topogra- (Fig. 10 ), typically during neap tides. Wetting and drying cycles have phy of highs and lows associated with signi ficant drainage channel been shown to vary considerably with elevation ( Fiot and Gratiot, Fig. 8. (a) Large-scale shore erosion and mangrove destruction during an inter-bank phase; (b) alternations of megacusps and embayments associated with large-scale coastal erosion; (c) substrate layering pattern following the erosion and retreat of a consolidated muddy mangrove substrate. Fresh mud may be deposited over the marsh surface but net retreat leads to scarping and the formation of mud pebbles that are visible above the freshly deposited mud; (c) from Lefebvre et al. (2004) .

2006 ). Changes in physical parameters, such as sediment erodibility, development were mainly controlled by elevation. Water loss water loss and pore-water salinity, indicated progressive mud flat occurred by drainage, modulated by the local tidal signal, with compaction as well as fluctuations related to the successive wetting weather conditions (notably temperature changes, and eventual and drying cycles ( Fiot and Gratiot, 2006 ). Mud cracks are features wetting by rainfall) playing a secondary role. that re flect the effects of contractional stresses ( Yesiller et al., 2000 ). Desiccation cracks can enable the trapping of floating propagules Mud cracks observed by Fiot and Gratiot (2006) were apparently of the mangrove Avicennia germinans as tides ebb ( Proisy et al., 2009 ). ephemeral features that (re)opened after a few days of dewatering Mangrove seedling establishment has been observed to be particu- and (re)healed during the subsequent wetting. Gardel et al. (2009) larly dependent on topographic changes, with very subtle elevation showed through a field experiment that included time-lapse changes in the upper intertidal zone (order of a few centimetres) photography, high-resolution topographic monitoring, collection of having a strong in fluence on successful colonization ( Fiot and Gratiot, meteorological data and measurements of the water contents of the 2006; Proisy et al., 2009 ). Under favourable elevation conditions, upper 30 cm of a mud bar, that consolidation of mud and mud-crack mangrove colonization rapidly ensues with plant densities exceeding MHW L

MWL

ML WL

Fig. 9. Cross-shore pro file across the Macouria mud-bank surface, compiled from the three methods depicted in Fig. 4 , synthesizing patterns of mud consolidation and mangrove colonization. Levels of consolidation are derived from field observations in the light of both published data ( Fiot and Gratiot, 2006 ) and unpublished data provided by Sandric Lesourd. Variations in relative mud consolidation in the lower intertidal zone (below MWL) re flect the preponderant role of wave remobilization and fluidization, while consolidation and mud concentration levels in the upper intertidal zone (above MWL) re flect both trapping of mud spilling over into troughs and depressions and in situ drying out and consolidation processes. From Anthony et al. (2008) .

30/m². Once colonization commences ( Fig. 10 ), extremely rapid experienced mangrove colonization. This embodies an apparent − mangrove growth (rates up to 2 m yr 1) leads to the establishment of contradiction, because intense wave forcing should lead to strong a fringe of young mangroves and mud stabilization ( Fig. 11 ). mud-bank mobilization. Possible explanations are: (1) the active Although large waves are expected to account for high rates of remobilization of mud by more energetic waves and its shoreward mud liquefaction and mobilization, Gardel and Gratiot (2005) showed migration towards the upper intertidal zone where mangrove that such waves might not necessarily have a destructive impact on colonization occurs; and (2) variations in wave-energy dissipation mangroves. Their analysis of shoreline changes in French Guiana over hinged on wave period. With cessation of wave forcing, remobilized the period 1995 –2000, characterized by high wave energy, showed mud forms bars that are the primary substrate for pioneer mangrove that mangroves in inter-bank areas underwent very active retreat formation. Since wave attenuation is frequency-dependent, wave − (150 to 200 m yr 1), but at the same time the mud-bank areas spectra are signi ﬁcantly distorted, as Jiang and Mehta (1996) showed

Fig. 10. Mud cracks and a diatom ﬁlm on a mud bank in French Guiana. Fig. 11. Rapid colonization by opportunistic Avicennia germinans mangroves of fresh mud translated across-shore during the onset of a bank phase in French Guiana. This freshly translated mud will progressively sti ﬂe the older mangroves in the background.

from field measurements on the seasonal shore-attached mud banks complementary field investigations in French Guiana, to de fine both of Kerala (India), and Winterwerp et al. (2007) in Surinam ( Fig. 6 ). event-scale and longer-term patterns of mud-bank migration. Thus, it is likely that swell energy is almost entirely expended on the Following the work of Rodriguez and Mehta (1998) , these authors 3 2 mobilization of mud bars while short waves propagate to the coast singled out the ratio H0/T , combining wave height ( H) and period ( T), with a signi ficant impact on inter-bank mangroves ( Gardel and and the angle of wave incidence ( α), as the most relevant parameters Gratiot, 2005 ). for describing wave forcing. Gratiot et al. (2007) showed that notable Reworking of the topographic highs inherited from the linear bars phases of increased wave energy were accompanied by higher annual by high-energy waves will likely affect mud dispersal over the rates of longshore mud-bank migration ( Fig. 12 ), but that the adjacent terrestrial mangrove substrates. Mud moved shoreward and correlation was rather poor between the wave forcing parameter 3 2 impinging on established mangrove swamps can lead to burial and H0/T and migration rates because of the contribution of other asphyxia of mangrove pneumatophores, resulting in the death of ‘old ’ mechanisms to bank migration. These are discussed next. mangrove trees ( Fromard et al., 1998, 2004 ). Adjacent to these areas Mud-bank migration rates can vary signi ficantly both alongshore are often found opportunistic rapid-growth juveniles ( Fig. 11 ) and in time, re flecting variability in bank and inter-bank dynamics. adapted to the new substrate topography ( Anthony et al., 2008 ). The banks in French Guiana exhibited low multi-annually averaged The relaxation of wave activity during the low wave-energy season migration rates (0.2 –1.8 km yr − 1) in the early 1980s and high rates − enables the subsequent survival of the young pioneer mangroves. On (1.8 –3.0 km yr 1) from the mid-1990s to 2005 ( Gardel and Gratiot, the seaward part of the nearshore pro file in the subtidal and lower 2005 ). The mean mud-bank migration rate from 1995 to 2000 was intertidal zones, wave reworking leaves behind a flat furrowed mud- twice that from 1979 to 1984, for instance, even though the wave 3 2 bank surface that will eventually be completely eroded as the narrow forcing parameter, H0/T , identi fied by Gratiot et al. (2007) from the trailing edge of the mud bank recedes towards this contact zone. 44-yr record of the ERA-40 wave dataset was only 33% higher. 3 2 Temporal changes in H0/T aside, there are several potential reasons 6. Mud-bank migration and medium-term (order of years) for these variations. These include unknown sediment sourcing shoreline dynamics aspects such as variations in mud supply from the Amazon and fluctuations in the temporal frame of mud-bank formation. Changes in Wave liquefaction of mud includes a longshore component that is the intensity and direction of the trade winds and their effects on fundamental to mud-bank migration. Following Wells and Coleman waves have been held responsible for temporal variability in mud- (1978, 1981a) , a number of theoretical efforts and a few field bank migration rates ( Eisma et al., 1991; Allison et al., 1995a, 2000 ). investigations on this and other mud-affected coasts have suggested Eisma et al. (1991) used the angle of incidence of winds as a surrogate a leading role for wind-generated waves in this process ( Jiang and for assessing temporal variations in the intensity of wave-generated Mehta, 1996; Rodriguez and Mehta, 1998, 2001; Chevalier et al., 2004; longshore drift, and, hence, mud-bank migration rates. This approach Tatavarti and Narayana, 2006; Gratiot et al., 2007; Chevalier et al., was further used by Augustinus (2004) to explain changes in the rates 2008 ). A 44-yr record (1960 –2004) of the ERA-40 wave dataset (see of mud-bank migration and the lengthening of mud banks in Surinam. also Fig. 5 ) was used by Gratiot et al. (2007) , together with Augustinus (2004) suggested that a more oblique orientation of a

c d

e f

Fig. 12. Mud-bank migration rates and wave dynamics in French Guiana. From Gratiot et al. (2007) : (a) longshore mud-bank migration rates between Cayenne and Kourou ( Fig. 1 ), from 1979 to 1983 (based on aerial photographic interpretations by Froidefond et al. (1988) ), and from 1992 to 2002 (based on satellite image interpretation by Gardel and Gratiot (2004, 2005) ); (b) bank and inter-bank mangrove shoreline evolution trends between Cayenne and Kourou from 1988 to 2002 (based on satellite image interpretation by Gardel and Gratiot (2005) ); (c), (d) inter-bank and mud-bank pro files and schematic wave attenuation patterns. MWL is the mean water level and MTR the mean tidal range deduced from tidal signal series; (e), (f) associated sediment surface concentration pro files; the circle diameter is representative of the vertical error bar. incident waves along the Surinam coast, related to a change in the (Fig. 8 b) that should affect wave-drift gradients alongshore. Another angle of the coast itself, compared to the French Guiana coast, source of variability is the response of mud-bank rheology to wave explained the larger mud-bank migration rates, and longer but less stress. The rheological behaviour of the mud shows a strongly non- wide mud banks (due to alongshore ‘stretching ’) on this coast. It may linear and thixotropic response to wave stress ( Fiot and Gratiot, be inferred from these observations that differences in migration rates 2006 ). Beyond a threshold forcing, the apparent mud viscosity may also account in part for variations in the spacing between the decreases considerably, and this could, in turn, strongly affect mudbanks. Temporal variations may also be generated by changes in bank migration rates. Variability in mud-bank migration rates must distant storm tracks and intensity patterns in the North Atlantic, such also be induced by a combination of other forcing mechanisms, as those associated with the North Atlantic Oscillation and El Niño and notably geostrophic forcing associated with the North Brazil Current La Niña events. (Nikiema et al., 2007 ), tidal currents propagating northwestwards Another set of factors involves local irregularities in the (Bourret et al., 2008 ), density currents due to the Amazon fresh water alongshore mud-bank migration corridor such as island and plume, the effect of impinging wind stress on the shore and the nearshore bedrock outcrops and rocky headlands in French Guiana generation of compensatory northwestward flows due to north to that trap mud ( Anthony and Dolique, 2004 ). River mouths and river northeasterly winds during the active trade-wind season. Currents discharge patterns have also been invoked as sources of migration- generated by wind stress would depend not only on wind velocities rate variability ( Gardel and Gratiot, 2005 ). Closely related to this set and incidence relative to the coast but also on shoreline morphology. of factors is the plan shape of the coast itself, which, in inter-bank These ancillary sources of hydrodynamic forcing provide scope for areas, may comprise the aforementioned megacusp-like alternations future studies. Evidence, for instance, for an overprint of the 18.6 yr nodal tidal 7.1. Mud-bank in fluence on embayed bedrock headland-bound sandy cycle on bank migration rates and attendant changes in shoreline beaches dynamics has been presented by Gratiot et al. (2008) . The demon- stration by Gratiot et al. (2008) concerned, in particular, the shoreline The only noteworthy sector where bedrock headlands indent the of French Guiana, and is based on 60 satellite images covering 39 dates muddy Amazon-Guianas coast is the 15 km-long coast of Cayenne. A from October 20, 1986 to January 15, 2006. A ‘typical ’ stretch of coast 500 m-long stretch of bedrock coast also occurs in Kourou, 35 km was analysed by Gratiot et al. (2008) , and consisted of five alternating west of Cayenne, both in French Guiana ( Fig. 1 ). The Cayenne sector regions of mangrove colonization (bank areas) and erosion (inter- differs from the rest of this fluctuating alluvial coast in that it bank areas) each 30 –40 km-long. These areas shifted northwestward comprises several headland-bound fringing sandy beaches rather and formed the fingerprints of mud banks migrating from Brazil to than cheniers. Sandy sedimentation on the Cayenne promontory has Surinam (at rates of 1 –3 km yr − 1). Gratiot et al. (2008) calculated been limited, notwithstanding the fact that the bedrock embayments from computation of topography from SPOT satellite images varia- offer accommodation space for potentially signi ficant sandy barrier tions in sediment balance that re flected this cyclic behaviour. Over the progradation. The limited progradation of these barriers is probably period 1988 –1999, the coastal sediment balance had a de ficit of due to the aforementioned sequestering of sand by the pervasive mud approximately 37×10 6 tons yr − 1, resulting in shoreline retreat. The on the shoreface and to the highly protruding nature of the Cayenne trend has reversed since 2000, with an estimated excess of promontory relative to the regional-scale mud-bank transport system 35×10 6 tons yr − 1 of shoreline sediment. These patterns were similar on the inner shoreface. This embayed bedrock coast differs, thus, in to those observed in neighbouring Surinam from 1966 to 1970 this regard from other swell or trade-wind wave-dominated embay- (Augustinus, 1978 ). The calculations carried out by Gratiot et al. ment-rich tropical and mid-latitude coasts, such as the southeastern (2008) showed that time slices of erosion and progradation initially coast of Australia ( Thom, 1984 ) and the coasts of Brazil ( Dominguez et identi fied by Choubert and Boyé (1959) appear to correlate with the al., 1992 ) and West Africa ( Anthony, 1995 ), where sandy barrier 18.6 yr nodal cycle. These results emphasise the plausibility of the progradation, commonly involving multiple beach ridges, has been hypothesis proposed by Wells and Coleman (1981b) on the role of important. The evidence from remote-sensing and field observations this cycle in generating periodic phases of higher high-tide water suggests that the headland-bound beaches and their associated levels. Such higher water levels are not only favourable to more narrow barrier accumulations in Cayenne have balanced long-term ef ficient wave-energy incidence and erosion of this muddy coast, but sand budgets. These beaches appear to have been sourced by sand may also impact the capacity of mangroves in colonizing higher parts supplied by the local rivers near Cayenne, as suggested by their rich of the mud banks by diminishing bank-surface exposure to desicca- heavy-mineral contents, and winnowed out by waves from ambient tion and the resulting development of mud cracks that favour such mud during ancestral inter-bank phases. colonization (see Section 5 ). Alternations between mud-bank phases and mud-poor inter-bank phases result in marked spatial and temporal variations in beach dynamics and morphology. The chief effect of the mud banks is to 7. Mud banks and sand bodies induce periodic alternations in longshore drift that lead to a form of beach ‘rotation ’, which is the periodic lateral movement of sand The Amazon-in fluenced coast receives variable amounts of sand towards alternating ends of an embayed beach ( Anthony et al., 2002; and mud from the local rivers west of the Amazon that drain the Anthony and Dolique, 2004 ). Rotation of French Guiana beaches does Quaternary coastal-plain and adjacent crystalline Guiana Shield. On not result from seasonal variations in deepwater wave approach this mud-dominated coast, sand is an important economical and directions, as is generally reported for rotating beaches not affected by ecological component because sandy deposits provide locations for mud banks (e.g., Ranasinghe et al., 2004; Short and Trembanis, 2004 ) human settlement. The rare beaches on this part of the South American or beaches subject to seasonal or episodic mud supply (e.g., Klein et coast are also fundamental for the ecology of protected leatherback al., 2002; Aubry et al., 2009; Calliari et al., 2009; Tamura et al., 2010 ). turtles ( Lepidochelys olivacea , Chelonia mydas , Eretmochelys imbricata , In French Guiana, beach rotation is due to short to medium-term Dermochelys coriacea ). These species require mud-free sandy beaches (order of a few years) changes in nearshore bathymetry induced by that are not subject to overwash by waves or tides for successful the migrating mud banks. These bathymetric changes affect wave nesting ( Kelle et al., 2007; Caut et al., 2010 ). Beach overwash and mud refraction and dissipation patterns, inducing strong longshore and organic matter have been shown by these authors to have gradients in waves. These strong wave-energy gradients along damaging effects on leatherback turtle nesting. sections of the shore facing the leading or trailing edges of the mud The limited presence of sand bodies on this coast re flects the banks generate lateral movement of sand in these headland-bound diluting in fluence of the enormous mud supply from the Amazon beaches, resulting in alternations of erosion and accretion areas over during the Quaternary and the limited sediment yield from the local, time ( Fig. 13 a) These beach morphological changes have been de fined well-forested drainage basins despite high rainfall. Blanketing of relict in terms of a simple, four-stage conceptual model comprising bank, fluvial sand by the cover of Amazon mud on the inner shelf has been inter-bank and transitional phases. Anthony and Dolique (2004, deemed to preclude shoreward sand reworking to form the beach and 2006) showed that dramatic short-term beach pro file oscillations of barrier systems typical of sand-rich wave-dominated shorefaces up to 100 m in two to three years ( Fig. 13 b) are strongly embedded in (Anthony and Dolique, 2004 ). Pujos et al. (2000) concluded from the large-scale mud-bank bathymetry-forced rotational process, analyses of the heavy-mineral assemblages of the quartz-dominated although smaller-scale self-organised behaviour may be involved in beach sands in French Guiana that these sands are derived exclusively bedform arrangements along the beach. Similar changes in hydrody- from local sources and not winnowed out from the migrating Amazon namic parameters with marked consequences on sandy beach mud banks. Coherent sand bodies present on the Amazon-in fluenced morphodynamics have recently been documented from field studies shores are rare. They are either bedrock-bound embayed beaches, on the periodically mud-fast Cassino beach adjacent to the Patos notably in the vicinity of Cayenne ( Fig. 1 ), in French Guiana, or, much estuary ( Calliari et al., 2009; Holland et al., 2009; Rogers and Holland, more commonly, cheniers ( Augustinus et al., 1989; Daniel, 1989; Prost, 2009 ). 1989 ). Cheniers can form signi ficant linear features, and are generally Mud welding onto the headland-bound sandy beaches of the found in eroding, inter-bank areas. In places, they are incorporated in Cayenne area may sequester sand eroded from these beaches. In such the prograded muddy coastal plain either as individual strands or as bank phases, mud directly welds onto the beaches for periods ranging bands of cheniers. from months to years, leading to a rare example where ocean-facing a Bank phase Transition

Atlantic Ocean

Longshore drift

Transition Interbank phase

18 15

Fig. 13. (a) A four-phase model of sandy beach morphological change involving rotation in response to Amazon mud-bank activity in Cayenne, French Guiana. Modified, from Anthony and Dolique, 2006 , with permission from John Wiley and Sons. The cycle comprises a bank, an inter-bank and two transitional phases, as a typical mud bank attains and migrates past the Cayenne headland. The transitional phase between bank and inter-bank phases shows the most rapid (days to months) and most spectacular beach changes because the natural longshore drift (from southeast to northwest) generated by trade-wind waves on this coast is reinforced by longshore gradients in wave height due to inshore dissipation by mud trapped by northwestern headlands. This occurs as the trailing edge of a mud bank goes past each headland-bound beach in Cayenne; (b) An example of the dramatic variations in the width of a sandy beach (Montjoly beach) subject to rotation induced by changes in wave parameters due to the impingement of a mud bank in Cayenne (from Anthony and Dolique, 2004 ). The pro file locations are shown in the inter-bank panel in (a). The changes cover a period of 21 months. beaches are completely incorporated into a temporarily prograded intertidal to shoreface muddy system. At the height of the bank phase, the muddy shores fronting the beaches become rapidly colonized by mangrove forests, sometimes over cross-shore distances of several hundreds of metres to several kilometres ( Anthony and Dolique, 2006 ). Subsequent mud erosion and mangrove forest destruction by the return of waves during inter-bank phases leads to the resumption of normal beach dynamics. This involves the restitution to the beach sand budget, of sand sequestered in the previous bank phase within shoreface mud deposits. Observations suggest that beach rotation does not affect the medium-term (order of tens of years) beach sand budgets. If illegal sand extractions to satisfy a growing building industry in Cayenne continue to go unchecked, they will impact these budgets. Beach rotation has been shown to be absent in very short (b150 m-long) pocket beaches on this muddy coast, because longshore gradients in sand transport do not develop ( Dolique and Anthony, 2005 ). In these pocket beaches, pro file oscillations are, thus, basically seasonal, and involve cross-shore sand mobility during wave-dominated inter-bank phases, but the pro file becomes muted during mud-dominated bank phases.

7.2. Cheniers

Where sand is locally available, and is concentrated by waves, inter-bank areas may be characterized by the active formation of cheniers ( Fig. 14 ). Such sand may also be released by the erosion of older cheniers incorporated in the prograded coastal plain. In the Amapà area of Brazil, at the approaches to the mouth of the Amazon, mud-bank formation is rather incipient (see Section 2 ), and minor amounts of sand supplied by the various small coastal rivers and/or winnowed out from mud are preferentially transported landward onto the mangrove fringe, producing very fine-grained (10 –12 φ mean grain size) accumulations ( Allison et al., 1995b; Allison and Nittrouer, 1998 ). These authors showed that sand bodies supplied by the local rivers in this area are composed of flaser beds, cross beds and massive beds, are up to 5 m-thick, and overlie an erosional mud shoreface. Due to the spatial pattern of mud-bank formation, which starts in the Cabo Cassipore area, these sand beaches form permanent features of the shoreline in this relatively mud-starved zone. Down- drift of this zone, there appears to be a clear gradient in the degree of chenier formation between the Brazil and French Guiana sectors on the one hand, and the Surinam sector on the other. In the former, current chenier formation and fossil cheniers within the prograded muddy plain are relatively rare, while more active chenier formation has prevailed in the latter sector, resulting in the incorporation of numerous bands of cheniers in the prograded Holocene coastal plain (Fig. 14 a). This alongshore gradient probably re flects the sand-supply in fluence of the much larger-sized rivers debouching from the granitic catchments of Surinam compared to the smaller sand-supply rivers in Brazil and French Guiana. Chenier development occurs through the concentration of sand by waves that are much less dissipated in inter-bank areas ( Augustinus, 1978 ). Chenier-forming processes are limited to high-tide phases, fi when wave-energy dissipation is less. The process is strongly Fig. 14. (a) ENVISAT image (2008) showing signi cant bands of linear cheniers on the prograded Holocene muddy plain of Surinam; (b) oblique air photograph depicting dominated by sand overwash over muddy, eroded, and commonly discrete sandy cheniers migrating shoreward over a muddy substrate comprising organic-rich, substrates ( Fig. 14 b,c). Where the shoreline exhibits a eroding rice fields in western French Guiana; (c) ground photographs showing wave megacuspate morphology, as outlined in Section 4.3 , cheniers are overwash at the back of a chenier. generally formed within the bays between consolidated and temporarily resistant muddy cuspate projections. theme that has recently been highlighted by Holland and Elmore (2008) . Sand concentration over muddy substrates has been observed 7.3. Sand –mud interactions and sand deformation structures to lead to the development of unique, but ephemeral, beach deformation and collapse features ( Anthony and Dolique, 2006 ). Notwithstanding the overwhelming in fluence of mud in the These features ( Fig. 15 a) appear to be part of the process of sand dynamics and evolution of the Amazon-in fluenced coast of South accumulation and adjustment to the underlying muddy substrate. America, the local presence of sand offers sediment heterogeneity, Although their development is hinged on the marked sedimentolog- and, therefore, marked divergence in coastal morphodynamics, a ical and geotechnical differences between the sand and mud, their Fig. 15. Beach collapse features due to subsidence of mud underlying a cover of sand in French Guiana. Modi fied, from Anthony and Dolique (2006) , with permission from John Wiley and Sons. (a) Schematic representation of stages of pro file subsidence and the formation of collapse features. (I) Plan view: SZ = stable updrift and downdrift beach zones; CZ = pro file collapse zone; ME = mud erosion; MD = mud deposition. The pro file collapse zone progressively shifts downdrift (arrow) with longshore sand transport, from a high (to the left of panel) to a low wave-energy zone (to the right of panel). (II) Schematic pro files of the various zones shown in (a). Small black vertical and white horizontal arrows in P2 (collapse zone) indicate subsidence and mud dewatering respectively. MHWS: Mean high water spring tide level; MLWS: Mean low water spring tide level. Note the inferred variations in the level of the mud surface (higher mud surface in low-energy P3 area of fresh mud accumulation — light and dark shadings of mud in pro file P3 represent, respectively freshly mobilized mud accumulating further downdrift in the MD zone, over older settled mud). Scales are approximate; (b) ground photographs showing collapse features. The collapse zone is preceded by a typical mud-erosion zone, the eroded mud accumulating further downdrift. Circled numbers 1 and 2 on photo II refer, respectively, to settled mud − − (density up to 1000 kg m 3) undergoing erosion, and to freshly accumulating fluid mud (density: 350 –600 kg m 3) derived from updrift erosion. formation is not due to hydraulic processes at the sand –mud interface, beach sands are often well packed. The linear nature of the cracks in such as sand piping or undermining, nor to collapse of void space such the sand and their strong development alongshore for tens of metres as from encapsulated air within the sand body, since chenier and in the mid- to lower beach zones on both beaches and cheniers and in this muddy environment are most likely explained by hydraulically- driven adjustment of the underlying mud to sand loading ( Fig. 15 b). Adjustment of the beach pro file to sand loading in the intertidal zone occurs through mud dewatering related to evaporation at low tide, when large areas of the foreshore are exposed, and to compaction of the underlying mud. These two processes generate accommodation space into which the overlying sand above the water ex filtration zone responds by forming subsiding packages of non-saturated sand delimited by cracks alongshore ( Anthony and Dolique, 2006 ). Piping processes are, however, well developed in the water ex filtration zone on the lower beach, and commonly generate additional deformation of the observed vertical collapse walls.

8. Discussion

8.1. The mud-bank –inter-bank cycle and overall coastal-plain progradation

The development of the shoreface and of the coastal plain between the Amazon and the Orinoco Rivers re flects both the high mud supply from the single Amazon source and a speci fic set of favourable oceanographic conditions prevailing on the shelf at the mouth of the Amazon and along the 1500 km-long coast of the Guianas. The high mud supply and the formation of an estuarine frontal zone associated with fresh water –salt water interaction over the shoreface lead to Fig. 16. A model (with high vertical exaggeration of the offshore slopes) for shoreline fl sea oor storage of much of the suspended sediment discharged by the evolution in French Guiana based on remote-sensing and field observations. From river, forming fluid-mud concentrations that are the precursors of Allison and Lee (2004) . The diagram shows a succession of nearshore cross-sections of mud banks. A complex combination of wave and tidal forcing and a stratigraphy (top to bottom) with the passage of an offshore mud bank. The eroded, pressure gradient set up by impinging onshore trade winds leads to relatively low-porosity inter-bank surface (top panel) is succeeded (second panel) by fl leading-edge mud-bank deposition in the subtidal zone and the upper intertidal zone uid-mud concentration in the Cabo Cassipore area of Brazil where driven by fluid mud delivered onshore during phases of coastal setup. Accretion mud banks start forming, before migrating along the coasts of the continues (third panel) in the upper intertidal zone as it translates seaward with Guianas. mangrove stabilization, but ceases offshore with passage of the leading edge of the mud Mud-bank migration involves spatio-temporal alternations of bank. With continued consolidation offshore (bottom panel), wave attack resumes and the coastal stratigraphic package is partially removed. This partial removal indicates bank and inter-bank phases that imply periodic wave recycling (at that there is a net coastal-plain growth with each mud-bank –inter-bank cycle. Note timescales of the order of years along any given stretch of shoreline) that sediments deposited in the intertidal zone can later be exposed in the inter-bank of muddy sediments and their relatively minor component of chenier subtidal zone (bottom panel). sands. Each inter-bank phase results in the partial, or rarely, total removal, of the coastal stratigraphic package ( Fig. 16 ). Total removal of the stratigraphic package deposited during a bank phase can occur Venezuela. The uppermost portion in this clinoform structure is the during a subsequent inter-bank phase characterized by particularly shoreline, the aggradation of which has brought the modern high wave-energy seasons such as during El Niño years ( Gratiot et al., sedimentary deposit to sea level ( Allison and Nittrouer, 1998 ). In 2008 ). More commonly, removal is partial, signifying that there is a the Amapà region in Brazil, shoreline aggradation of 5 –10 m has net coastal-plain growth with each cycle ( Allison and Lee, 2004 ). occurred, and the shoreline deposits are prograding across topset Where sand has been locally available or concentrated by wave action, strata of the modern subaqueous delta, which is the lowermost and individual cheniers, or bands of cheniers in sand-rich contexts, have most important part of the compound Holocene clinoform structure been incorporated into the prograded coastal plain. The only (Allison et al., 1995b; Allison and Nittrouer, 1998; Walsh and exceptions to this progradational context are in the Cayenne and Nittrouer, 2009 ). In places, the onshore clinoform is much thicker, Kourou areas in French Guiana where highly projecting bedrock but may exhibit a complex pattern of dissection by estuarine channels. headlands with embayed beaches still prevail, and where mud-bank – The subaqueous delta extends to a water depth of 70 m, with a inter-bank cycles have not resulted in coastal progradation but are rollover point of maximum slope change (e.g., between topset and expressed as marked spatio-temporal beach morphodynamic alter- foreset strata) at 30 –40 m. Advective sediment input to the foreset − nations ( Fig. 15 ). In these areas, narrow fringing beaches respond region causes very high accumulation rates ( N10 cm yr 1), which morphodynamically to the mud-bank –inter-bank cycle by rapid control the geometry and progradation of the clinoform (order of months) changes in pro file due to cross-shore mud and structure. Sommer field et al. (2004) provided stratigraphic evidence sand mobility, and, in the longer embayed beaches, to marked of multi-secular ( b1 ka) late Holocene changes in shelf sedimentation gradients in longshore sand drift involving periodic rotation. Having at the mouth of the Amazon related to fluvial, oceanographic and been devoid of fresh sand sourcing, these beaches show balanced meteorologic processes, and that are independent of the in fluence of sediment budgets over the Holocene timescale. sea-level variations. Erosional phases on the shelf during the Holocene appear to correspond to phases of signi ficant shoreline storage of mud 8.2. Muddy shoreline progradation and large-scale clinoform involving rapid coastal progradation, as outlined previously. development Downdrift of the mouth of the Amazon, coastal progradation is expressed by the overlapping of northward-extending mud capes that Over geological timescales, the high mud supply from the Amazon are well developed where river mouths debouch and are diverted has generated a Holocene clinoform structure that attains its northwestward ( Fig. 1 ). The overall progradational sequence com- maximum development off the mouth of the river, and more or less prises the three distinct dynamic/morphosedimentary types dis- important coastal progradation throughout the Guianas coast into cussed in the preceding sections: accretional mud, accretional sand and erosional mud ( Allison et al., 1995b ), the last two types being longshore or cross-shore dispersal. This coast is fronted by long sandy associated with mud-de ficient and/or inter-bank areas. It is expected beaches and lacks rivers liable to supply mud. Narayana et al. (2008) that in chenier-forming areas along the Guianas coast, this prograda- have suggested that the Kerala mud banks are palimpsest, marshy, tional mud wedge includes thin strands of chenier sands re flecting lagoonal deposits rich in organic matter and derived gas, that were periodic reworking, and integration within the prograding muddy submerged after a marine transgression. The mud-bank regime on the plain during inter-bank phases, of shoreface sands delivered by the Kerala coast of India has been reported to be an essentially in situ smaller coastal rivers. phenomenon in fluenced by seasonal monsoonal wave-energy varia- In areas of low or no progradation such as offshore of Cayenne, the tions. High-energy monsoonal waves are responsible for the formation of flapping mud sequences thin seaward, merging with a relict shelf and sustenance of the mud banks, and the seabed returns to its pre- surface that is overlain further seaward by modern shelf mud monsoon state during the low-energy season. Although some fine (Bouysse et al., 1977; Pujos et al., 1990; Allison and Nittrouer, sediment disperses alongshore and offshore, most is returned to the 1998 ). In the French Guiana area, the progradational wedge comprises seabed as the monsoon declines. Wave refraction over the shoreface over-consolidated mud on the inner shelf (0 to −20 m) that has bathymetry may lead to shore-fast mud-bank formation at known yielded radiocarbon ages ranging from over 40 ka to present ( Cleac'h, locations of wave-energy concentration, but such mud is restored back 1999 ). This over-consolidated mud forms, thus, a relict Pleistocene to to the shoreface mud-bank reservoir during the non-monsoon season Holocene bed surface ( Bouysse et al., 1977; Pujos et al., 1990 ) over (Mathew and Baba, 1995 ). which the mud banks comprising fluid and under-consolidated mud At the geological timescale, signi ficant mud supply can also lead, as migrate ( Allison et al., 2000 ), within a dynamic system of short-term in the case of the Amazon, to the development of thick muddy coastal to seasonal sedimentation and resuspension cycles. This dynamic clinoforms under conditions of progradation associated with still- system feeds the long-term inner shelf mud accumulation, with stand conditions. A fine example is that of the Gulf of Papua ( Walsh et offshore thinning of the mud wedge being dependent on waves, as on al., 2004 ). Allison and Nittrouer (1998) have suggested similarities wave-dominated shelves. Seismic data from the shelf deposits show between the Mississippi –Atchafalaya clinoform system ( Neill and that the thin inner shelf mud overlies sandy deposits of presumably Allison, 2005 ) and that of the Amazon-in fluenced coast, although a fluvial origin that crop out on the middle shelf at depths beyond 20 m storm-dominated sedimentary signature leads to more complex (Bouysse et al., 1977; Pujos et al., 1990 ). Onshore in Surinam, the arrangements in the former, as McBride et al. (2007) have shown. similarity of the Pleistocene deposits with the modern mud-bank Similar complexity is evinced by Asian deltaic systems (e.g., Hori et al., system, both in terms of alternations between bank and chenier-rich 2004; van Maren, 2005 ) where storm events, variations in river inter-bank sequences and of the forcing mechanisms in coastal discharge, and delta channel switching lead to signi ficant interlacing sedimentation, has been highlighted by Wong et al. (2009) . of muddy and sandy deposits. Clinoform structures in high mud-supply shores such as that of the 8.3. Comparisons with other muddy coasts Amazon-in fluenced coast of South America provide analogues of the geological record created there ( Wong et al., 2009 ) and elsewhere for Other notable examples of open muddy coasts in the world are muddy shorefaces ( Ginsburg, 2005 ). Depending on regional char- commonly associated with river-dominated deltas, and include the acteristics (e.g. more rapid subsidence), large fractions of clinoform Texas –Louisiana coast to the west of the Mississippi, and numerous structures could be preserved ( Nittrouer et al., 1995b ). A possible high-discharge rivers in Asia. Such shores are generally flanked by candidate for a fossil example of the Amazon-type shoreface is that of marshes or mangroves and bare mud flats several hundreds of metres the Alderson Member formation in western Canada ( Hovikoski et al., to several kilometres wide. Although such systems may be character- 2008 ). Ginsburg (2005) considers as other possible candidates the ized by widespread mud dispersal from the river point sources (e.g., inner shelf and shoreline parts of the thick Tertiary deltaic deposits of Wright and Nittrouer, 1995 ), mud-bank formation and migration are the Gulf Coast of the US and the mud rock sequence of the Devonian not an overarching characteristic of these muddy coasts. The mud- Catskill Delta of New York. There are many other examples of similar bank system of the Amazon –Guianas coast appears to be unique in ancient analogues in the Cretaceous western interior seaway of the terms of both the magnitude of mud migration alongshore and the western United States. These ancient analogues, however, particularly mud dynamics, due to a combination of extremely large and pervasive of the shoreface clinoform, may probably be misinterpreted as deep mud supply to the shoreface. This sediment supply involves mud basinal marine deposits due to their homogeneous mud sections and concentration processes within an atypical shoreface-based estuarine near-absence of fossils. turbidity maximum, and conservative onshore-alongshore mud transport along an energetic inner shoreface belt stretching for over 9. Conclusions 1500 km from the mouth of the Amazon to that of the Orinoco. Mud- bank systems on other mud-rich coasts, such as the Jaba and Purari The large supply of mud from the Amazon has led to the growth of coasts of Indonesia ( Wright, 1989 ), the West African coast between an important muddy clinoform off the mouth of this river and to rapid Guinea –Bissau and Sierra Leone ( Anthony, 1989, 2004, 2006; Capo et muddy coastal progradation incorporating chenier sands over more al., 2006 ), the Gulf of Papua ( Wolanski and Alongi, 1995 ), the Cassino than 1500 km of coast northwest of the Amazon. Coastal progradation coast of Brazil ( Calliari et al., 2002 ), and the Kerala coast of India has occurred through the shoreward translation of mud from mud (Narayana et al., 2008 ) are much smaller and some, such as those of the banks migrating alongshore. Mud-bank formation, migration and Cassino coast, do not demonstrate longshore migration. Some of these interactivity with the shore are associated with speci fic water-column examples also concern mud-bank formation and activity associated strati fication and wave-dampening processes that determine cross- with numerous adjacent river mouths, as in the Indonesian and West shore and longshore mud dynamics. These processes also have an African examples. The ensuing mud-bank regime is, however, similar overwhelming impact on the ecology and economy of this coast. to that of the Amazon in that the estuarine turbidity maxima of the Research on the mechanisms of muddy shoreline progradation has various longshore-connected rivers develop at the open river mouths been signi ficantly aided by remote-sensing techniques and comple- or over a shallow shoreface under high seasonal river discharge, and mented by field measurements, notably on mechanisms of shoreward the mud banks migrate alongshore under the in fluence of seasonal mud advection by waves, and on the ensuing mud deposition rates wind-waves and currents. The mud-bank system on the coast of and mud-bank topography. This original sequence of mud supply, Kerala, in India, is different, and appears to comprise self-organised concentration, formation, and migration involves processes that forms that undergo a seasonal cycle of dynamic changes involving no require further study. Among themes of particular interest are: (1) the timescales and interactions involved in the formation of mud Anthony, E.J., Dolique, F., 2006. Intertidal subsidence and collapse features on wave- – exposed, drift-aligned sandy beaches subject to Amazon mud: Cayenne, French banks; (2) wave mud bed interactions and their modulation by Guiana. Earth Surf. Proc. Land. 31, 1051 –1057. differences in wave characteristics and mud consolidation; (3) Anthony, E.J., Gardel, A., Dolique, F., Guiral, D., 2002. Short-term changes in the plan processes and timescales of longshore mud-bank migration and the shape of a sandy beach in response to sheltering by a nearshore mud bank, Cayenne, French Guiana. Earth Surf. Proc. Land. 27, 857 –866. role of mud-supply, oceanographic and geological factors in inducing Anthony, E.J., Dolique, F., Gardel, A., Gratiot, N., Proisy, C., Polidori, L., 2008. Nearshore variations in migration rates the impacts of which are important in intertidal topography and topographic-forcing mechanisms of an Amazon-derived terms of ecosystem-based coastal management; and (4) the meso- mud bank in French Guiana. Cont. Shelf Res. 28, 813 –822. scale to long-term patterns of coastal erosion and progradation, and Aubry, A., Lesourd, S., Gardel, A., Dubuisson, P., Jeanson, M., 2009. Sediment textural variability and mud storage on a large accreting sand flat in a macrotidal, storm- the way these are archived in the coastal-plain sediments from the wave setting: the North Sea coast of France. J. Coast. Res. SI 56, 163 –167. Amazon to the Orinoco, especially at the Holocene timescale. These Augustinus, P.G.E.F., 1978. The changing shoreline of Surinam (South America). Ph.D. fi Thesis, Univ. Utrecht. 232 pp. themes offer scope for signi cant future research involving innovative fl fi Augustinus, P.G.E.F., 2004. The in uence of the trade winds on the coastal development eld and remote-sensing approaches. The dynamics and deposits on of the Guianas at various scale levels: a synthesis. Mar. Geol. 208, 141 –151. these shores could also serve as archives of conditions prevailing in Augustinus, P.G.E.F., Hazelhoff, L., Kroon, A., 1989. The chenier coast of Suriname: – the Amazon basin and yield insight on the geological past. modern and geological development. Mar. Geol. 90, 269 281. Ba ghdadi, N., Oliveros, C., 2007. Potential of ASAR/Envisat data for mud bank monitoring in French Guiana compared to ASTER imagery. J. Coast. Res. 23, 1509 –1517. Acknowledgements Baghdadi, N., Gratiot, N., Lefebvre, J.P., Oliveros, C., Bourguignon, A., 2004. Coastline and mudbank monitoring in French Guiana: contributions of radar and optical satellite imagery. Can. J. Remote Sens. 30, 109 –122. We thank John Walsh and two anonymous reviewers for their Baltzer, F., Allison, M.A., Fromard, F., 2004. Material exchange between the continental salient suggestions. Denis Marin is thanked for preparing the shelf and mangrove-fringed coasts with special reference to the Amazon –Guianas fi coast. Mar. Geol. 208, 113 –114. illustrations. Authors E.J.A., A.G., N.G., C.P., F.D., and F.F. have bene ted Bourret, A., Devenon, J.L., Chevalier, C., 2008. Tidal in fluence on the hydrodynamics of from various French research grants over the years involving work on the French Guiana continental shelf. Cont. Shelf Res. 28, 951 –961. the Amazon-in fluenced coast of South America. These include funding Bouysse, P., Kudrass, H.R., Le Lann, F., 1977. Reconnaissance sédimentologique du by the Institut National des Sciences de l'Univers (INSU), by the plateau continental de la Guyane française (mission Guyamer 1975). Bull. Bur. Rech. Géol. Min. 4, 141 –179. Agence National de la Recherche (ANR), by the Centre National Calliari, L.J., Speranski, N.S., Torronteguy, M., Oliveira, M.B., 2002. The mud banks of d'Etudes Spatiales (CNES), and by the Université du Littoral Côte Cassino beach, southern Brazil: characteristics, processes and effects. J. Coast. Res. – d'Opale (ULCO). M.A.A. bene fited from grants by the American SI 34, 318 325. Calliari, L.J., Winterwerp, J.C., Fernandes, E., Cuchiara, D., Vinzon, S.B., Sperle, M., National Science Foundation and the American Chemical Society Holland, K.T., 2009. Fine grain sediment transport and deposition in the Patos Petroleum Research Fund. During many of the field missions, logistical Lagoon –Cassino beach sedimentary system. Cont. Shelf Res. 29, 515 –529. support was provided by the Institut de Recherche pour le Capo, S., Sottolichio, A., Brenon, I., Castaing, P., Ferry, L., 2006. Morphology, hydrography and sediment dynamics in a mangrove estuary: the Konkouré estuary, Guinea. Mar. Développement (IRD) in Cayenne. Special thanks in this regard to Geol. 230, 199 –215. Daniel Guiral of IRD. Caut, S., Guirlet, E., Girondot, M., 2010. Effect of tidal overwash on the embryonic development of leatherback turtles in French Guiana. Mar. Environ. Res. 69, 254 –261. Chevalier, C., Baklouti, M., Ramamonjiarisoa, A., 2004. Modeling the in fluence of wind References and rivers on current, salinity and temperature over the French Guiana continental shelf during the rainy season. J. Coast. Res. 20, 1183 –1197. Aller, R.C., Heilbrun, C., Panzeca, C., Zhu, Z., Baltzer, F., 2004. Coupling between Chevalier, C., Froidefond, J.M., Devenon, J.L., 2008. Numerical analysis of the combined sedimentary dynamics, early diagenetic processes, and biogeochemical cycling in action of littoral current, tide and waves on the suspended mud transport and on the Amazon –Guianas mobile mud belt: coastal French Guiana. Mar. Geol. 208, turbid plumes around French Guiana mudbanks. Cont. Shelf Res. 28, 545 –560. 331 –360. Choubert, B., Boyé, M., 1959. Envasements et désenvasements du littoral en Guyane Aller, J.Y., Aller, R.C., Kemp, P.F., Christoserdov, A., Madrid, V.M., 2010. Fluidized muds: a française. C.R. Acad. Sci. 249, 145 –147. novel setting for biosphere diversity through geological time. Geobiology 8, Cleac'h, F., 1999. Les vasières du Quaternaire terminal sur les plateaux continentaux du 169 –178. Suriname et du Guyana (Amérique du Sud): modalités de formation et Aller, R.C., Madrid, V., Christoserdov, A., Aller, J.Y., Heilbrun, C., 2010. Understanding d'alimentation. Mémoire de D.E.A., Université de Bordeaux I. diagenetic processes and sulfur biogeochemistry in tropical deltaic muds: Coco, G., Murray, A.B., 2007. Patterns in the sand: from forcing templates to self- implications for oceanic isotope cycles and the sedimentary record. Geochim. organization. Geomorphology 91, 271 –290. Cosmochim. Acta 74, 4671 –4692. Dagg, M., Benner, R., Lohrenz, S., Lawrence, D., 2004. Transformation of dissolved and Allison, M.A., Lee, M.T., 2004. Sediment exchange between Amazon mudbanks and particulate materials on continental shelves in fluenced by large rivers: plume fringing mangroves in French Guiana. Mar. Geol. 208, 169 –190. processes. Cont. Shelf Res. 24, 833 –858. Allison, M.A., Nittrouer, C.A., 1998. Identifying accretionary mud shorefaces in the Daniel, J.R.K., 1989. The chenier plain coastal system of Guyana. Mar. Geol. 90, 283 –287. geologic record: insights from the modern Amazon dispersal system. In: Scheiber, De Wit, P.J., Kranenberg, C., 1997. On the liquefaction and erosion of mud due to waves J., Zimmerle, W., Sethi, P. (Eds.), Shales and Mudstones. I. E. Schwezerbart'sche and current. In: Burt, N., Parker, R., Watts, J. (Eds.), Cohesive Sediments. John Wiley Verlagbuchhandlung, Stuttgart, pp. 147 –161. & Sons, pp. 331 –340. Allison, M.A., Nittrouer, C.A., Faria, L.E.C., 1995a. Sources and sinks of sediment to the Debenay, J.P., Jouanneau, J.M., Sylvestre, F., Weber, O., Guiral, D., 2007. Biological origin Amazon margin: the Amapà coast. Geo-Mar. Lett. 16, 36 –40. of rhythmites in muddy sediments of French Guiana. J. Coast. Res. 23, 1431 –1442. Allison, M.A., Nittrouer, C.A., Kineke, G.C., 1995b. Seasonal sediment storage on Dolique, F., Anthony, E.J., 2005. Short-term pro file changes of sandy pocket beaches mud flats adjacent to the Amazon River. Mar. Geol. 125, 303 –328. affected by Amazon-derived mud, Cayenne, French Guiana. J. Coast. Res. 21, Allison, M.A., Nittrouer, C.A., Faria, L.E.C., Mendes, E.C., 1996. Rates and mechanisms of 1195 –1202. shoreface progradation and retreat downdrift of the Amazon River mouth. Dominguez, J.M.L., Bittencourt, A.C.S.P., Martin, L., 1992. Controls on Quaternary coastal Allison, M.A., Lee, M.T., Ogston, A.S., Aller, R.C., 2000. Origin of Amazon mudbanks along evolution of the east-northeastern coast of Brazil: roles of sea-level history, trade the northeastern coast of South America. Mar. Geol. 163, 241 –256. winds and climate. Sed. Geol. 80, 213 –232. Anthony, E.J., 1989. Chenier plain development in northern Sierra Leone, West Africa. Dunne, T., Mertes, L.A.K., Meade, R., Richey, J.E., Forsberg, B.R., 1998. Exchanges of Mar. Geol. 90, 297 –309. sediment between the floodplain and channel of the Amazon River in Brazil. Geol. Anthony, E.J., 1995. Beach-ridge progradation in response to sediment supply: Soc. Am. Bull. 110, 450 –467. examples from West Africa. Mar. Geol. 129, 175 –186. Eisma, D., Augustinus, P.G.E.F., Alexander, C.R., 1991. Recent and subrecent changes in Anthony, E.J., 2004. Sediment dynamics and morphological stability of an estuarine the dispersal of Amazon mud. Netherlands. J. Sea Res. 28, 181 –192. mangrove complex: Sherbro Bay, West Africa. Mar. Geol. 208, 207 –224. Filizola, N., 2003. Transfert sédimentaire actuel par les fleuves amazoniens. Ph.D. Thesis, Anthony, E.J., 2006. The muddy tropical coast of West Africa from Sierra Leone to Paul Sabatier University, Toulouse, 292 pp. Guinea –Bissau: geological heritage, geomorphology and sediment dynamics. Afr. Fiot, J., Gratiot, N., 2006. Structural effects of tidal exposure on mud flats along the Geosci. Rev. 13, 227 –237. French Guiana coast. Mar. Geol. 228, 25 –37. Anthony, E.J., 2009. Shore processes and their palaeoenvironmental applications. Froidefond, J.M., Pujos, M., Andre, X., 1988. Migration of mud-banks and changing Developments in Marine Geology, Volume 4. Elsevier Science, Amsterdam. 519 pp. coastline in French Guiana. Mar. Geol. 84, 19 –30. Anthony, E.J., Dolique, F., 2004. Morphological response of embayed sandy beaches to Froidefond, J.M., Lahet, F., Hu, C., Doxaran, D., Guiral, D., Prost, M.T., Ternon, J.F., 2004. Amazon-derived mud banks, Cayenne, French Guiana: a short- to long-term Mud flats and mud suspension observed from satellite data in French Guiana. Mar. perspective. Mar. Geol. 208, 249 –264. Geol. 208, 153 –168. Fromard, F., Puig, H., Mougin, E., Marty, G., Betoulle, J.L., Cadamuro, L., 1998. Structure, Mehta, A.J., 2002. Mudshore dynamics and controls. In: Healy, T., Wang, Y., Healy, J.A. above-ground biomass and dynamics of mangrove ecosystems: new data from (Eds.), Muddy Coasts of the World: Processes, Deposits and Function. : Proceedings French Guiana. Oecologia 115, 39 –53. in Marine Science, 4. Elsevier, pp. 19 –60. Fromard, F., Vega, C., Proisy, C., 2004. Half a century of dynamic coastal change affecting Milliman, J.D., Meade, R.H., 1983. World-wide delivery of river sediment to the oceans. mangrove shorelines of French Guiana. A case study based on remote sensing data J. Geol. 91, 1 –21. analyses and field surveys. Mar. Geol. 208, 265 –280. Molleri, G.S.F., de M. Novo, E.M.L., Kampel, M., 2010. Space –time variability of the Gardel, A., Gratiot, N., 2004. Monitoring of coastal dynamics in French Guiana from Amazon River plume based on satellite ocean color. Cont. Shelf Res. 30, 342 –352. 16 years of SPOT images. J. Coast. Res. SI 39, 1502 –1505. Narayana, A.C., Jago, C.F., Manojkumar, P., Tatavarti, R., 2008. Nearshore sediment Gardel, A., Gratiot, N., 2005. A satellite image-based method for estimating rates of mud characteristics and formation of mudbanks along the Kerala coast, southwest India. bank migration, French Guiana, South America. J. Coast. Res. 21, 720 –728. Estuar. Coast. Shelf Sci. 78, 341 –352. Gardel, A., Proisy, C., Lesourd, S., Philippe, S., Caillaud, J., Gontharet, S., Anthony, E.J., Brutier, NEDECO, 1968. Suriname Transportation Study: Report on Hydraulic Investigation. L., 2009. A better understanding of mud cracking processes gained from in situ Netherlands Engineering Consultants, The Hague. 293 pp. measurements on an intertidal mud flat in French Guiana. J. Coast. Res. SI 56, 424 –428. Neill, C.F., Allison, M.A., 2005. Subaqueous deltaic formation on the Atchafalaya Shelf, Geyer, W.R., Kineke, G.C., 1995. Observations of currents and water properties in the Louisiana. Mar. Geol. 214, 411 –430. Amazon frontal zone. J. Geophys. Res. 100, 2321 –2339. Nikiema, O., Devenon, J.L., Baklouti, M., 2007. Numerical modelling of the Amazon River Geyer, W.R., Beardsley, R.C., Lentz, S.J., Candela, J., Limeburner, K., Johns, W.E., Castro, B. plume. Cont. Shelf Res. 27, 873 –899. M., Soares, I.D., 1996. Physical oceanography of the Amazon shelf. Cont. Shelf Res. Nittrouer, C.A., Demaster, D.J., 1986. Sedimentary processes on the Amazon continental 16, 576 –616. shelf — past, present and future research. Cont. Shelf Res. 6, 5 –30. Geyer, W.R., Hill, P.S., Kineke, G.C., 2004. The transport, transformation and dispersal of Nittrouer, C.A., Kuehl, S.A., 1995. Geological signi ficance of sediment transport and sediment by buoyant coastal flows. Cont. Shelf Res. 24, 927 –949. accumulation on the Amazon continental shelf — preface. Mar. Geol. 125, 175 –176. Ginsburg, R.N., 2005. Disobedient sediments can feedback on their transportation, Nittrouer, C.A., Kuehl, S.A., Sternberg, R.W., Figueiredo, A.G., Faria, L.E.C., 1995a. An deposition and geomorphology. Sed. Geol. 175, 9 –18. introduction to the geological signi ficance of sediment transport and accumulation Gratiot, N., Gardel, A., Anthony, E.J., 2007. Trade-wind waves and mud dynamics on the on the Amazon continental shelf. Mar. Geol. 125, 177 –192. French Guiana coast, South America: input from ERA-40 wave data and field Nittrouer, C.A., Kuehl, S.A., Figueiredo, A.G., Allison, M.A., Sommer field, C.K., Rine, J.M., investigations. Mar. Geol. 236, 15 –26. Faria, L.E.C., Silveira, O.M., 1995b. The geological record preserved by Amazon shelf Gratiot, N., Anthony, E.J., Gardel, A., Gaucherel, C., Proisy, C., Wells, J.T., 2008. Signi ficant sedimentation. Cont. Shelf Res. 16, 817 –841. contribution of the 18.6 year tidal cycle to regional coastal changes. Nat. Geosci. 1, Organization of American States, 2005. Integrated and Sustainable Management of 169 –172. Transboundary Water Resources in the Amazon River Basin,.Water Project Series 8. Guyot, J.L., Walling, D.E. (Eds.), 2009. Flow and Sediment Dynamics of Large Rivers: 6 pp. www.oas.org/osde . Hydrol. Proc., Special Issue, 22, pp. 3127 –3270. Proisy, C., Couteron, F., Fromard, F., 2007. Predicting and mapping mangrove biomass Holland, K.T., Elmore, P.A., 2008. A review of heterogeneous sediments in coastal from canopy grain analysis using Fourier-based textural ordination of IKONOS environments. Earth Sci. Rev. 89, 116 –134. images. Remote Sens. Environ. 109, 379 –392. Holland, K.T., Vinzon, S.B., Calliari, L.J., 2009. A field study of coastal dynamics on a Proisy, C., Gratiot, N., Anthony, E.J., Gardel, A., Fromard, F., Heuret, P., 2009. Mud bank muddy coast offshore of Cassino Beach, Brazil. Cont. Shelf Res. 29, 503 –514. colonization by opportunistic mangroves: a case study from French Guiana using Hori, K., Tanabe, S., Saito, Y., Haruyama, S., Nguyen, V., Kitamura, A., 2004. Delta lidar data. Cont. Shelf Res. 29, 632 –641. initiation and Holocene sea-level change: example from the Song Hong (Red River) Prost, M.T., 1989. Coastal dynamics and chenier sands in French Guiana. Mar. Geol. 90, delta, Vietnam. Sed. Geol. 164, 237 –249. 259 –267. Hovikoski, J., Lemiski, R., Gingras, M., Pemberton, G., MacEachern, J.A., 2008. Ichnology Pujos, M., Bouysse, P., Pons, J.C., 1990. Sources and distribution of heavy minerals in Late and sedimentology of a mud-dominated Alderson Member (Lea Park FM), Western Quaternary sediments of the French Guiana continental shelf. Cont. Shelf Res. 10, Canada. J. Sed. Res. 78, 803 –824. 59 –79. Jaramillo, S., Sheremet, A., Allison, M.A., Holland, K.T., Reed, A.H., 2009. Wave –mud Pujos, M., Pons, J.C., Parra, M., 2000. Les minéraux lourds des sables du littoral de la interactions over the muddy Atchafalaya subaqueous clinoform, Louisiana, United Guyane française: bilan sur l'origine des dépôts de la plate-forme des Guyanes. States: wave-supported sediment transport. J. Geophys. Res. Oceans 114, C04002 Oceanol. Acta 24, S27 –S35 (Supplement). doi:10.1029/2008JC004821 . Ranasinghe, R., McLoughlin, R., Short, A., Symonds, G., 2004. The Southern Oscillation Jiang, F., Mehta, A.J., 1996. Mud banks of the southwest coast of India. V: Wave Index, wave climate, and beach rotation. Mar. Geol. 204, 273 –287. attenuation. J. Coast. Res. 12, 890 –897. Rine, J.M., Ginsburg, R.N., 1985. Depositional facies of a mud shoreface in Suriname, Kelle, L., Gratiot, N., Nolibos, L., Therese, J., Wongsopawiro, R., De Thoisy, B., 2007. South America: a mud analogue to sandy, shallow-marine deposits. J. Sed. Petrol. Monitoring of nesting leatherback turtles ( Dermochelys coriacea ): contribution of 55, 633 –652. remote-sensing for real time assessment of beach coverage in French Guiana. Rodriguez, H.N., Mehta, A.J., 1998. Considerations on wave-induced fluid mud Chelonian Conserv. Biol. 6, 142 –149. streaming at open coasts. In: Black, K.S., Paterson, D.M., Cramp, A. (Eds.), Kineke, G.C., Sternberg, R.W., Trowbridge, J.H., Geyer, W.R., 1996. Fluid-mud processes Sedimentary Processes in the Intertidal Zone: Geological Society, London, Special on the Amazon continental shelf. Cont. Shelf Res. 16, 667 –696. Publications, 139, pp. 177 –186. Kirby, R., 2000. Practical implications of tidal flat shape. Cont. Shelf Res. 20, 1061 –1077. Rodriguez, H.N., Mehta, A.J., 2001. Modelling of muddy coast response to waves. J. Coast. Kirby, R., 2002. Distinguishing accretion from erosion-dominated muddy coasts. In: Res. SI 21, 132 –148. Healy, T., Wang, Y., Healy, J.A. (Eds.), Muddy Coasts of the World: Processes, Rogers, W.E., Holland, K.T., 2009. A study of dissipation of wind-waves by mud at Deposits and Function. : Proceedings in Marine Science. Elsevier, pp. 61 –81. Cassino Beach, Brazil: Prediction and inversion. Cont. Shelf Res. 29, 676 –690. Klein, A.H., Filho, L.B., Schumacher, D.H., 2002. Short-term beach rotation processes in Sheremet, A., Stone, G.W., 2003. Observations of nearshore wave dissipation over distinct headland bay beach systems. J. Coast. Res. 18, 442 –458. muddy sea beds. J. Geophys. Res. 108 (C11), 21-1/21-11. Kuehl, S.A., Nittrouer, C.A., Allison, M.A., Faria, L.E.C., Dukat, D.A., Jaeger, J.M., Pacioni, T. Short, A.D., Trembanis, A.C., 2004. Decadal scale patterns in beach oscillation and D., Figueiredo, A.G., Underkof fler, E.C., 1996. Sediment deposition, accumulation, rotation Narrabeen Beach, Australia — time series, PCA and wavelet analysis. J. and seabed dynamics in an energetic fine-grained coastal environment. Cont. Shelf Coast. Res. 20, 523 –532. Res. 16, 787 –815. Sommer field, C.K., Nittrouer, C.A., Figueredo, A.G., 2004. Stratigraphic evidence of Lakhan, V.C., Pepper, D.A., 1997. Relationship between concavity and convexity of a changes in Amazon shelf sedimentation during the late Holocene. Mar. Geol. 125, coast and erosion and accretion patterns. J. Coast. Res. 13, 226 –232. 351 –371. Lambs, L., Muller, E., Fromard, F., 2007. The Guianese paradox: how can freshwater Strasser, M.A., Vinzon, S.B., Kosuth, P., 2002. Bottom structures geometry of the Amazon out flow from the Amazon increase the salinity of the Guianan shore? J. Hydrol. 342, River. International Conference on Fluvial Hydraulics, Louvain-la-Neuve, pp. 1185 –1193. 88 –96. Tamura, T., Horaguchi, K., Saito, Y., Nguyen, V.L., Tateishi, M., Ta, T.K.O., Nanayama, F., Lefebvre, J.P., Dolique, F., Gratiot, N., 2004. Geomorphic evolution of a coastal mud flat Watanabe, K., 2010. Monsoon-in fluenced variations in morphology and sediment under oceanic in fluences: an example from the dynamic shoreline of French of a mesotidal beach on the Mekong River delta coast. Geomorphology 116, 11 –23. Guiana. Mar. Geol. 208, 191 –205. Tatavarti, R., Narayana, A.C., 2006. Hydrodynamics in a mud bank regime during Liu, J.P., Li, A.C., Xu, K.H., Velozzi, D.M., Yang, Z.S., Milliman, J.D., DeMaster, D.J., 2006. nonmonsoon and monsoon seasons. J. Coast. Res. 22, 1463 –1473. Sedimentary features of the Yangtze River-derived alongshelf clinoform deposit in Thom, B.G., 1984. Sand barriers of eastern Australia: Gippsland — a case study. In: Thom, B. the East China Sea. Cont. Shelf Res. 26, 2141 –2156. G. (Ed.), Coastal Geomorphology in Australia. Academic Press, Sydney, pp. 233 –261. Martinez, J.M., Guyot, J.L., Filizola, N., Sondag, F., 2009. Increase in sediment discharge of Trowbridge, J.H., Kineke, G.C., 1994. Structure and dynamics of fluid muds on the the Amazon River assessed by monitoring network and satellite data. Catena 79, Amazon continental shelf. J. Geophys. Res. Oceans 99 (C1), 865 –874. 257 –264. van Ledden, M., Vaughn, G., Lansen, J., Wiersma, F., Amsterdam, M., 2009. Extreme Mathew, J., Baba, M., 1995. Mudbanks of the southwest coast of India II: wave mud waves along the Guyana coastline in October 2005. Cont. Shelf Res. 29, 352 –361. interactions. J. Coast. Res. 11, 179 –187. van Maren, D.S., 2005. Barrier formation on an actively prograding delta system: the Mathew, J., Baba, M., Kurian, N.P., 1995. Mud banks of the southwest coast of India I: Red River Delta, Vietnam. Mar. Geol. 224, 123 –143. wave characteristics. J. Coast. Res. 11, 168 –178. Walsh, J.P., Nittrouer, C.A., 2004. Mangrove-bank sedimentation in a mesotidal McBride, R.A., Taylor, M.J., Byrnes, M.R., 2007. Coastal morphodynamics and chenier- environment with large sediment supply, Gulf of Papua. Mar. Geol. 208, 225 –248. plain evolution in southwestern Louisiana, USA: a geomorphic model. Geomor- Walsh, J.P., Nittrouer, C.A., 2009. Understanding fine-grained river-sediment dispersal phology 88, 367 –422. on continental margins. Mar. Geol. 263, 34 –45. Meade, R.H., Dunne, T., Richey, J.E., Santos, U.M., Salati, E., 1985. Storage and Walsh, J.P., Nittrouer, C.A., Palinkas, C.M., Ogston, A.S., Sternberg, R.W., Brunskill, G.J., remobilization of suspended sediment in the lower Amazon River of Brazil. Science 2004. Clinoform mechanics in the Gulf of Papua, New Guinea. Cont. Shelf Res. 24, 228, 488 –490. 2487 –2510. Warne, A.G., Meade, R.H., White, W.A., Guevara, E.H., Gigeaut, J., Smyth, R.C., Aslan, A., Winterwerp, J.C., de Graaff, R.F., Groeneweg, J., Luijendijk, A.P., 2007. Modelling of wave Tremblay, T., 2002. Regional controls on geomorphology, hydrology and ecosystem damping at Guyana mud coast. Coast. Eng. 54, 249 –261. integrity in the Orinoco Delta, Venezuela. Geomorphology 44, 273 –307. Wolanski, E., Alongi, D.M., 1995. A hypothesis for the formation of a mud bank in the Wei, T., Chen, Z., Duan, L., Gu, J., Saito, Y., Zhang, W., Wang, Y., Kanai, Y., 2007. Gulf of Papua. Geo-Mar. Lett. 15, 166 –171. Sedimentation rates in relation to sedimentary processes of the Yangtze Estuary, Wong, Th.E., de Kramer, R., de Boer, P.L., Langereis, C., Sew-A-Tjon, J., 2009. The China. Estuar. Coast. Shelf Sci. 71, 37 –46. in fluence of sea level changes on tropical coastal wetlands: the Pleistocene Wells, J.T., 1983. Dynamics of coastal fluid muds in low-, moderate-, and high-tide- Coropina formation, Suriname. Sed. Geol. 216, 127 –137. range environments. Can. J. Fish. Aquat. Sci. 40, 130 –142. Wright, L.D., 1989. Dispersal and deposition of river sediment in coastal seas — models Wells, J.T., Coleman, J.M., 1978. Longshore transport of mud by waves: northeastern from Asia and the Tropics. Netherlands. J. Sea Res. 23, 493 –500. coast of South America. Geol. Mijnbouw 57, 353 –359. Wright, L.D., Nittrouer, C.A., 1995. Dispersal of river sediment in coastal seas — 6 Wells, J.T., Coleman, J.M., 1981a. Physical processes and fine-grained sediment contrasting cases. Estuaries 18, 494 –508. dynamics, coast of Surinam, South America. J. Sed. Petrol. 51, 1053 –1068. Yesiller, N., Miller, C.J., Inci, G., Yaldo, K., 2000. Desiccation and cracking behavior of Wells, J.T., Coleman, J.M., 1981b. Periodic mud flat progradation, northeastern coast of three compacted land fill liner soils. Eng. Geol. 57, 105 –121. South America: a hypothesis. J. Sed. Petrol. 51, 1053 –1068. Zhu, Z., Aller, R.C., Mak, J., 2002. Stable carbon isotope cycling in mobile coastal muds of Wells, J.T., Kemp, G.P., 1986. Interaction of surface waves and cohesive sediments: field Amapà, Brazil. Cont. Shelf Res. 22, 2065 –2079. observations and geologic signi ficance. In: Mehta, A.J. (Ed.), Lecture Notes on Coastal and Estuarine Studies, Estuarine Cohesive Sediment Dynamics, 14, pp. 43 –65.